From 549cc3a2459553f53b92f5ced4a9fbab4aa16329 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Tue, 19 May 2026 14:16:33 -0700
Subject: [PATCH 01/22] Try to fix issue

---
 r/R/arrow-package.R             |  5 +++++
 r/src/safe-call-into-r-impl.cpp | 11 ++++++++++-
 2 files changed, 15 insertions(+), 1 deletion(-)

diff --git a/r/R/arrow-package.R b/r/R/arrow-package.R
index 2706faee5cb1..c5b3008a6ca9 100644
--- a/r/R/arrow-package.R
+++ b/r/R/arrow-package.R
@@ -150,6 +150,11 @@ s3_finalizer <- new.env(parent = emptyenv())
   # needs the C++ library loaded
   create_binding_cache()
 
+  if (identical(R.version$os, "emscripten")) {
+    # Disable multithreading on WASM/Emscripten (no pthread support)
+    options(arrow.use_threads = FALSE)
+  }
+
   if (tolower(Sys.info()[["sysname"]]) == "windows") {
     # Disable multithreading on Windows
     # See https://issues.apache.org/jira/browse/ARROW-8379
diff --git a/r/src/safe-call-into-r-impl.cpp b/r/src/safe-call-into-r-impl.cpp
index c2fa1e1eac6b..bb3530cb0021 100644
--- a/r/src/safe-call-into-r-impl.cpp
+++ b/r/src/safe-call-into-r-impl.cpp
@@ -45,7 +45,16 @@ bool SetEnableSignalStopSource(bool enabled) {
 }
 
 // [[arrow::export]]
-bool CanRunWithCapturedR() { return MainRThread::GetInstance().Executor() == nullptr; }
+bool CanRunWithCapturedR() {
+#ifdef __EMSCRIPTEN__
+  // Threading is not supported under Emscripten/WASM. Always take the
+  // synchronous path to avoid attempting pthread_create which will fail
+  // with "thread constructor failed: Not supported".
+  return false;
+#else
+  return MainRThread::GetInstance().Executor() == nullptr;
+#endif
+}
 
 // [[arrow::export]]
 std::string TestSafeCallIntoR(cpp11::function r_fun_that_returns_a_string,

From 4162c550976a9578d153c7b380173e6e175a3f76 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Tue, 19 May 2026 14:44:01 -0700
Subject: [PATCH 02/22] test not setting usethreads

---
 r/R/arrow-package.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/r/R/arrow-package.R b/r/R/arrow-package.R
index c5b3008a6ca9..c46b15fb226d 100644
--- a/r/R/arrow-package.R
+++ b/r/R/arrow-package.R
@@ -152,7 +152,7 @@ s3_finalizer <- new.env(parent = emptyenv())
 
   if (identical(R.version$os, "emscripten")) {
     # Disable multithreading on WASM/Emscripten (no pthread support)
-    options(arrow.use_threads = FALSE)
+    # options(arrow.use_threads = FALSE)
   }
 
   if (tolower(Sys.info()[["sysname"]]) == "windows") {

From 67ca35fd4df1078f1a0bdae8ad5a9e32425999d7 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Tue, 19 May 2026 20:52:42 -0700
Subject: [PATCH 03/22] set back

---
 r/R/arrow-package.R | 4 ++--
 1 file changed, 2 insertions(+), 2 deletions(-)

diff --git a/r/R/arrow-package.R b/r/R/arrow-package.R
index c46b15fb226d..721165ab114e 100644
--- a/r/R/arrow-package.R
+++ b/r/R/arrow-package.R
@@ -151,8 +151,8 @@ s3_finalizer <- new.env(parent = emptyenv())
   create_binding_cache()
 
   if (identical(R.version$os, "emscripten")) {
-    # Disable multithreading on WASM/Emscripten (no pthread support)
-    # options(arrow.use_threads = FALSE)
+    # Disable multithreading on Wasm/Emscripten
+    options(arrow.use_threads = FALSE)
   }
 
   if (tolower(Sys.info()[["sysname"]]) == "windows") {

From 29e79374d8e090ba7d7b6e583953b36c0a3648b7 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 20 May 2026 12:39:48 -0700
Subject: [PATCH 04/22] wip, run tests under wasm

---
 ci/scripts/r_wasm_test.cjs          | 141 ++++++++++++++++++++++++++++
 ci/scripts/r_wasm_test.sh           |  80 ++++++++++++++++
 dev/tasks/r/github.linux.r-wasm.yml |  11 ++-
 3 files changed, 231 insertions(+), 1 deletion(-)
 create mode 100644 ci/scripts/r_wasm_test.cjs
 create mode 100755 ci/scripts/r_wasm_test.sh

diff --git a/ci/scripts/r_wasm_test.cjs b/ci/scripts/r_wasm_test.cjs
new file mode 100644
index 000000000000..3700268326a7
--- /dev/null
+++ b/ci/scripts/r_wasm_test.cjs
@@ -0,0 +1,141 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements.  See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership.  The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License.  You may obtain a copy of the License at
+//
+//   http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+// Smoke-test and run the testthat suite for the arrow R package under webR.
+//
+// This script is called by r_wasm_test.sh after it sets up the CRAN-like
+// repo and installs the webr npm package.
+//
+// Environment variables:
+//   ARROW_WASM_REPO_DIR - path to the local CRAN-like repo containing
+//                         the arrow wasm binary package
+
+const { WebR } = require("webr");
+const http = require("http");
+const fs = require("fs");
+const path = require("path");
+
+const repoDir = process.env.ARROW_WASM_REPO_DIR;
+if (!repoDir) {
+  console.error("ERROR: ARROW_WASM_REPO_DIR not set");
+  process.exit(1);
+}
+
+async function main() {
+  // Serve the local repo over HTTP so webR (Emscripten) can access it.
+  // webR's R runs in an Emscripten sandbox and cannot access the host
+  // filesystem directly — it fetches packages over HTTP instead.
+  const server = http.createServer((req, res) => {
+    const filePath = path.join(repoDir, decodeURIComponent(req.url));
+    fs.readFile(filePath, (err, data) => {
+      if (err) {
+        res.writeHead(404);
+        res.end();
+      } else {
+        res.writeHead(200);
+        res.end(data);
+      }
+    });
+  });
+  server.listen(8080);
+  console.log("✓ Repo server on :8080");
+
+  const webR = new WebR({
+    RArgs: ["--quiet"],
+    interactive: false,
+  });
+
+  await webR.init();
+  console.log("✓ webR initialized");
+
+  // Install the arrow Wasm package, put localhost:8080 before repo.r-wasm.org
+  // (which is used for deps)
+  await webR.installPackages(["arrow"], {
+    repos: ["http://localhost:8080", "https://repo.r-wasm.org"],
+    quiet: false,
+    mount: false,
+  });
+  console.log("✓ arrow installed");
+
+  // Install test deps. TOOD: This is flakey. We could parse the DESCRIPTION
+  // file to be more robust.
+  await webR.installPackages(
+    ["testthat", "tibble", "dplyr", "withr", "pillar"],
+    {
+      repos: ["https://repo.r-wasm.org"],
+      quiet: false,
+      mount: false,
+    },
+  );
+  console.log("✓ test dependencies installed");
+
+  // Test the package loads and functions basically
+  const loadResult = await webR.evalRString(`
+    library(arrow)
+    cat("arrow loaded\\n")
+    cat("R.version$os =", R.version$os, "\\n")
+    use_threads <- getOption("arrow.use_threads")
+    cat("arrow.use_threads =", use_threads, "\\n")
+    stopifnot(identical(use_threads, FALSE))
+    tab <- arrow::as_arrow_table(data.frame(x = 1:10, y = letters[1:10]))
+    stopifnot(nrow(tab) == 10L)
+    cat("Created Arrow table with", nrow(tab), "rows\\n")
+    "PASS"
+  `);
+
+  if (loadResult !== "PASS") {
+    console.error("Package load test FAILED");
+    await webR.close();
+    server.close();
+    process.exit(1);
+  }
+  console.log("✓ Package loads and works correctly");
+
+  // Run tests
+  console.log("Running testthat suite under webR...");
+
+  const testResult = await webR.evalRString(`
+    library(testthat)
+    library(arrow)
+    results <- testthat::test_package("arrow", reporter = "summary", stop_on_failure = FALSE)
+    df <- as.data.frame(results)
+    n_pass <- sum(df$passed)
+    n_skip <- sum(df$skipped)
+    n_fail <- sum(df$failed)
+    n_error <- sum(df$error)
+    cat(sprintf("Results: %d passed, %d skipped, %d failed, %d errors\\n",
+                n_pass, n_skip, n_fail, n_error))
+    if (n_fail > 0 || n_error > 0) "FAIL" else "PASS"
+  `);
+
+  if (testResult !== "PASS") {
+    console.error("testthat suite FAILED");
+    await webR.close();
+    server.close();
+    process.exit(1);
+  }
+  console.log("✓ testthat suite passed");
+
+  console.log("✓ All tests passed!");
+  await webR.close();
+  server.close();
+}
+
+main().catch((e) => {
+  console.error("FAILED:", e);
+  process.exit(1);
+});
diff --git a/ci/scripts/r_wasm_test.sh b/ci/scripts/r_wasm_test.sh
new file mode 100755
index 000000000000..80191785afcb
--- /dev/null
+++ b/ci/scripts/r_wasm_test.sh
@@ -0,0 +1,80 @@
+#!/usr/bin/env bash
+# Licensed to the Apache Software Foundation (ASF) under one
+# or more contributor license agreements.  See the NOTICE file
+# distributed with this work for additional information
+# regarding copyright ownership.  The ASF licenses this file
+# to you under the Apache License, Version 2.0 (the
+# "License"); you may not use this file except in compliance
+# with the License.  You may obtain a copy of the License at
+#
+#   http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing,
+# software distributed under the License is distributed on an
+# "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+# KIND, either express or implied.  See the License for the
+# specific language governing permissions and limitations
+# under the License.
+
+# Test the arrow R package built for WebAssembly.
+#
+# This script is intended to run inside the ghcr.io/r-universe-org/build-wasm
+# Docker container after rwasm::build() has produced a .tgz binary. It:
+#   1. Sets up a CRAN-like repo structure from the built .tgz
+#   2. Installs the npm webr package (Node.js webR runtime)
+#   3. Boots webR, installs arrow from the local repo, and verifies:
+#      - The package can be installed and loaded
+#      - Multithreading is disabled (arrow.use_threads == FALSE)
+#      - The testthat test suite runs
+#
+# Tests that require threading are automatically skipped via
+# skip_if_not(CanRunWithCapturedR()) since CanRunWithCapturedR() returns
+# FALSE under Emscripten.
+#
+# Usage:
+#   r_wasm_test.sh <path-to-arrow-r-dir>
+#
+# Example:
+#   r_wasm_test.sh /work
+#
+# The arrow .tgz file(s) should already exist in <path-to-arrow-r-dir>.
+
+set -euxo pipefail
+
+SCRIPT_DIR="$(cd "$(dirname "$0")" && pwd)"
+arrow_r_dir="${1:-.}"
+
+# Set up a fake CRAN-like repo so we can install the package
+tgz_file=$(ls "${arrow_r_dir}"/arrow_*.tgz 2>/dev/null | head -1)
+if [ -z "${tgz_file}" ]; then
+  echo "ERROR: No arrow_*.tgz found in ${arrow_r_dir}" >&2
+  exit 1
+fi
+echo "Found Wasm binary: ${tgz_file}"
+
+repo_dir=$(mktemp -d)
+# TODO: Not sure if we need this
+# Cover multiple R minor versions in case the npm webr package
+# uses a different R version than the Docker image's build R.
+for r_ver in 4.4 4.5 4.6; do
+  contrib_dir="${repo_dir}/bin/emscripten/contrib/${r_ver}"
+  mkdir -p "${contrib_dir}"
+  cp "${tgz_file}" "${contrib_dir}/"
+  # type=mac.binary matches .tgz file extension
+  R -q -e "tools::write_PACKAGES('${contrib_dir}', type = 'mac.binary')"
+done
+
+echo "Repo structure:"
+find "${repo_dir}" -type f
+
+# Install webr in a temporary node project
+work_dir=$(mktemp -d)
+cd "${work_dir}"
+npm init -y > /dev/null 2>&1
+npm install --silent webr 2>/dev/null
+
+# Run our test script
+ARROW_WASM_REPO_DIR="${repo_dir}" node "${SCRIPT_DIR}/r_wasm_test.cjs"
+
+# Cleanup temp dirs
+rm -rf "${work_dir}" "${repo_dir}"
diff --git a/dev/tasks/r/github.linux.r-wasm.yml b/dev/tasks/r/github.linux.r-wasm.yml
index ee38740bb47d..542be91d9e7b 100644
--- a/dev/tasks/r/github.linux.r-wasm.yml
+++ b/dev/tasks/r/github.linux.r-wasm.yml
@@ -21,7 +21,7 @@
 
 jobs:
   r-universe-wasm:
-    name: "R-universe Wasm build"
+    name: "R-universe Wasm build and test"
     runs-on: ubuntu-latest
     timeout-minutes: 60
 
@@ -56,6 +56,15 @@ jobs:
                 2>&1 | tee build-wasm.log
             '
 
+      - name: Smoke-test arrow in webR
+        shell: bash
+        run: |
+          docker run --rm \
+            -v "${PWD}/arrow:/arrow" \
+            -w /tmp \
+            ghcr.io/r-universe-org/build-wasm:latest \
+            bash /arrow/ci/scripts/r_wasm_test.sh /arrow/r
+
       - name: List generated artifacts
         if: always()
         shell: bash

From e821b9181cc450e36fb00a6951742a400776cde4 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 20 May 2026 16:28:12 -0700
Subject: [PATCH 05/22] set NODE_PATH

---
 ci/scripts/r_wasm_test.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/ci/scripts/r_wasm_test.sh b/ci/scripts/r_wasm_test.sh
index 80191785afcb..f09324cc31ba 100755
--- a/ci/scripts/r_wasm_test.sh
+++ b/ci/scripts/r_wasm_test.sh
@@ -74,7 +74,7 @@ npm init -y > /dev/null 2>&1
 npm install --silent webr 2>/dev/null
 
 # Run our test script
-ARROW_WASM_REPO_DIR="${repo_dir}" node "${SCRIPT_DIR}/r_wasm_test.cjs"
+ARROW_WASM_REPO_DIR="${repo_dir}" NODE_PATH="${work_dir}/node_modules" node "${SCRIPT_DIR}/r_wasm_test.cjs"
 
 # Cleanup temp dirs
 rm -rf "${work_dir}" "${repo_dir}"

From c43659ef1790a4009190bcd5d61ff3445bede752 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 20 May 2026 20:33:17 -0700
Subject: [PATCH 06/22] try this for tests

---
 ci/scripts/r_wasm_test.cjs | 69 ++++++++++++++++++++++++++++++++++----
 ci/scripts/r_wasm_test.sh  |  2 +-
 2 files changed, 63 insertions(+), 8 deletions(-)

diff --git a/ci/scripts/r_wasm_test.cjs b/ci/scripts/r_wasm_test.cjs
index 3700268326a7..34226e173578 100644
--- a/ci/scripts/r_wasm_test.cjs
+++ b/ci/scripts/r_wasm_test.cjs
@@ -21,8 +21,9 @@
 // repo and installs the webr npm package.
 //
 // Environment variables:
-//   ARROW_WASM_REPO_DIR - path to the local CRAN-like repo containing
-//                         the arrow wasm binary package
+//   ARROW_WASM_REPO_DIR  - path to the local CRAN-like repo containing
+//                          the arrow wasm binary package
+//   ARROW_R_TESTS_DIR   - path to the arrow R package tests/testthat directory
 
 const { WebR } = require("webr");
 const http = require("http");
@@ -35,6 +36,26 @@ if (!repoDir) {
   process.exit(1);
 }
 
+const testsDir = process.env.ARROW_R_TESTS_DIR;
+if (!testsDir) {
+  console.error("ERROR: ARROW_R_TESTS_DIR not set");
+  process.exit(1);
+}
+
+// Recursively list all files under a directory
+function listFilesRecursive(dir) {
+  const results = [];
+  for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
+    const full = path.join(dir, entry.name);
+    if (entry.isDirectory()) {
+      results.push(...listFilesRecursive(full));
+    } else {
+      results.push(full);
+    }
+  }
+  return results;
+}
+
 async function main() {
   // Serve the local repo over HTTP so webR (Emscripten) can access it.
   // webR's R runs in an Emscripten sandbox and cannot access the host
@@ -62,6 +83,25 @@ async function main() {
   await webR.init();
   console.log("✓ webR initialized");
 
+  // Upload test files from the host into webR's virtual filesystem.
+  // rwasm builds don't include tests (no --install-tests flag), so we
+  // load them separately and use test_dir() instead of test_package().
+  const vfsTestDir = "/tmp/arrow-tests";
+  await webR.FS.mkdir(vfsTestDir);
+  const testFiles = listFilesRecursive(testsDir);
+  for (const filePath of testFiles) {
+    const rel = path.relative(testsDir, filePath);
+    const vfsPath = path.posix.join(vfsTestDir, rel.split(path.sep).join("/"));
+    // Ensure parent directories exist
+    const vfsDir = path.posix.dirname(vfsPath);
+    if (vfsDir !== vfsTestDir) {
+      await webR.evalRVoid(`dir.create("${vfsDir}", recursive = TRUE, showWarnings = FALSE)`);
+    }
+    const contents = fs.readFileSync(filePath);
+    await webR.FS.writeFile(vfsPath, contents);
+  }
+  console.log(`✓ Uploaded ${testFiles.length} test files to webR VFS`);
+
   // Install the arrow Wasm package, put localhost:8080 before repo.r-wasm.org
   // (which is used for deps)
   await webR.installPackages(["arrow"], {
@@ -71,10 +111,25 @@ async function main() {
   });
   console.log("✓ arrow installed");
 
-  // Install test deps. TOOD: This is flakey. We could parse the DESCRIPTION
-  // file to be more robust.
+  // Install test deps from DESCRIPTION Suggests + packages used in helpers.
+  // All of these are available on repo.r-wasm.org for wasm.
   await webR.installPackages(
-    ["testthat", "tibble", "dplyr", "withr", "pillar"],
+    [
+      "testthat",
+      "tibble",
+      "dplyr",
+      "withr",
+      "pillar",
+      "lubridate",
+      "purrr",
+      "stringr",
+      "stringi",
+      "bit64",
+      "hms",
+      "rlang",
+      "vctrs",
+      "tzdb",
+    ],
     {
       repos: ["https://repo.r-wasm.org"],
       quiet: false,
@@ -105,13 +160,13 @@ async function main() {
   }
   console.log("✓ Package loads and works correctly");
 
-  // Run tests
+  // Run tests using test_dir() since the wasm binary doesn't include tests
   console.log("Running testthat suite under webR...");
 
   const testResult = await webR.evalRString(`
     library(testthat)
     library(arrow)
-    results <- testthat::test_package("arrow", reporter = "summary", stop_on_failure = FALSE)
+    results <- testthat::test_dir("${vfsTestDir}", reporter = "summary", stop_on_failure = FALSE, package = "arrow")
     df <- as.data.frame(results)
     n_pass <- sum(df$passed)
     n_skip <- sum(df$skipped)
diff --git a/ci/scripts/r_wasm_test.sh b/ci/scripts/r_wasm_test.sh
index f09324cc31ba..d50a15d4e52b 100755
--- a/ci/scripts/r_wasm_test.sh
+++ b/ci/scripts/r_wasm_test.sh
@@ -74,7 +74,7 @@ npm init -y > /dev/null 2>&1
 npm install --silent webr 2>/dev/null
 
 # Run our test script
-ARROW_WASM_REPO_DIR="${repo_dir}" NODE_PATH="${work_dir}/node_modules" node "${SCRIPT_DIR}/r_wasm_test.cjs"
+ARROW_WASM_REPO_DIR="${repo_dir}" ARROW_R_TESTS_DIR="${arrow_r_dir}/tests/testthat" NODE_PATH="${work_dir}/node_modules" node "${SCRIPT_DIR}/r_wasm_test.cjs"
 
 # Cleanup temp dirs
 rm -rf "${work_dir}" "${repo_dir}"

From a5787ad5ac33c8a7f53f65a5b194783afd5a956a Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 20 May 2026 20:38:44 -0700
Subject: [PATCH 07/22] tidy up a bit

---
 ci/scripts/r_wasm_test.cjs | 134 ++++++++++++++-----------------------
 1 file changed, 49 insertions(+), 85 deletions(-)

diff --git a/ci/scripts/r_wasm_test.cjs b/ci/scripts/r_wasm_test.cjs
index 34226e173578..054119039861 100644
--- a/ci/scripts/r_wasm_test.cjs
+++ b/ci/scripts/r_wasm_test.cjs
@@ -15,15 +15,10 @@
 // specific language governing permissions and limitations
 // under the License.
 
-// Smoke-test and run the testthat suite for the arrow R package under webR.
-//
-// This script is called by r_wasm_test.sh after it sets up the CRAN-like
-// repo and installs the webr npm package.
-//
-// Environment variables:
-//   ARROW_WASM_REPO_DIR  - path to the local CRAN-like repo containing
-//                          the arrow wasm binary package
-//   ARROW_R_TESTS_DIR   - path to the arrow R package tests/testthat directory
+// Smoke-test the arrow R package under webR, then run the testthat suite.
+// Called by r_wasm_test.sh. Requires env vars:
+//   ARROW_WASM_REPO_DIR - local CRAN-like repo with the arrow .tgz
+//   ARROW_R_TESTS_DIR   - path to tests/testthat in the source tree
 
 const { WebR } = require("webr");
 const http = require("http");
@@ -42,7 +37,6 @@ if (!testsDir) {
   process.exit(1);
 }
 
-// Recursively list all files under a directory
 function listFilesRecursive(dir) {
   const results = [];
   for (const entry of fs.readdirSync(dir, { withFileTypes: true })) {
@@ -57,9 +51,7 @@ function listFilesRecursive(dir) {
 }
 
 async function main() {
-  // Serve the local repo over HTTP so webR (Emscripten) can access it.
-  // webR's R runs in an Emscripten sandbox and cannot access the host
-  // filesystem directly — it fetches packages over HTTP instead.
+  // Serve the repo over HTTP (webR can't access the host filesystem directly)
   const server = http.createServer((req, res) => {
     const filePath = path.join(repoDir, decodeURIComponent(req.url));
     fs.readFile(filePath, (err, data) => {
@@ -75,35 +67,28 @@ async function main() {
   server.listen(8080);
   console.log("✓ Repo server on :8080");
 
-  const webR = new WebR({
-    RArgs: ["--quiet"],
-    interactive: false,
-  });
-
+  const webR = new WebR({ RArgs: ["--quiet"], interactive: false });
   await webR.init();
   console.log("✓ webR initialized");
 
-  // Upload test files from the host into webR's virtual filesystem.
-  // rwasm builds don't include tests (no --install-tests flag), so we
-  // load them separately and use test_dir() instead of test_package().
+  // Upload test files to webR VFS (rwasm doesn't include tests in binaries)
   const vfsTestDir = "/tmp/arrow-tests";
   await webR.FS.mkdir(vfsTestDir);
   const testFiles = listFilesRecursive(testsDir);
-  for (const filePath of testFiles) {
-    const rel = path.relative(testsDir, filePath);
+  const createdDirs = new Set([vfsTestDir]);
+  for (const file of testFiles) {
+    const rel = path.relative(testsDir, file);
     const vfsPath = path.posix.join(vfsTestDir, rel.split(path.sep).join("/"));
-    // Ensure parent directories exist
     const vfsDir = path.posix.dirname(vfsPath);
-    if (vfsDir !== vfsTestDir) {
-      await webR.evalRVoid(`dir.create("${vfsDir}", recursive = TRUE, showWarnings = FALSE)`);
+    if (!createdDirs.has(vfsDir)) {
+      await webR.evalRVoid(`dir.create("${vfsDir}", recursive=TRUE, showWarnings=FALSE)`);
+      createdDirs.add(vfsDir);
     }
-    const contents = fs.readFileSync(filePath);
-    await webR.FS.writeFile(vfsPath, contents);
+    await webR.FS.writeFile(vfsPath, fs.readFileSync(file));
   }
-  console.log(`✓ Uploaded ${testFiles.length} test files to webR VFS`);
+  console.log(`✓ Uploaded ${testFiles.length} test files to VFS`);
 
-  // Install the arrow Wasm package, put localhost:8080 before repo.r-wasm.org
-  // (which is used for deps)
+  // Install arrow from local repo, deps from r-wasm.org
   await webR.installPackages(["arrow"], {
     repos: ["http://localhost:8080", "https://repo.r-wasm.org"],
     quiet: false,
@@ -111,81 +96,60 @@ async function main() {
   });
   console.log("✓ arrow installed");
 
-  // Install test deps from DESCRIPTION Suggests + packages used in helpers.
-  // All of these are available on repo.r-wasm.org for wasm.
-  await webR.installPackages(
-    [
-      "testthat",
-      "tibble",
-      "dplyr",
-      "withr",
-      "pillar",
-      "lubridate",
-      "purrr",
-      "stringr",
-      "stringi",
-      "bit64",
-      "hms",
-      "rlang",
-      "vctrs",
-      "tzdb",
-    ],
-    {
-      repos: ["https://repo.r-wasm.org"],
-      quiet: false,
-      mount: false,
-    },
-  );
+  // Install test deps parsed from DESCRIPTION
+  const depsList = await webR.evalRString(`
+    desc <- read.dcf(system.file("DESCRIPTION", package = "arrow"),
+                     fields = c("Imports", "Suggests"))
+    pkgs <- unlist(strsplit(paste(na.omit(desc[1,]), collapse = ","), ",\\\\s*"))
+    pkgs <- trimws(sub("\\\\s*\\\\(.*\\\\)", "", pkgs))
+    pkgs <- pkgs[pkgs != "" & pkgs != "R"]
+    pkgs <- pkgs[!pkgs %in% loadedNamespaces()]
+    paste(pkgs, collapse = "\\n")
+  `);
+  const testDeps = depsList.split("\n").filter(Boolean);
+  console.log(`Installing ${testDeps.length} dependencies from DESCRIPTION...`);
+  await webR.installPackages(testDeps, {
+    repos: ["https://repo.r-wasm.org"],
+    quiet: false,
+    mount: false,
+  });
   console.log("✓ test dependencies installed");
 
-  // Test the package loads and functions basically
+  // Smoke test: package loads, threading disabled, basic operations work
   const loadResult = await webR.evalRString(`
     library(arrow)
-    cat("arrow loaded\\n")
     cat("R.version$os =", R.version$os, "\\n")
-    use_threads <- getOption("arrow.use_threads")
-    cat("arrow.use_threads =", use_threads, "\\n")
-    stopifnot(identical(use_threads, FALSE))
+    stopifnot(identical(getOption("arrow.use_threads"), FALSE))
     tab <- arrow::as_arrow_table(data.frame(x = 1:10, y = letters[1:10]))
     stopifnot(nrow(tab) == 10L)
-    cat("Created Arrow table with", nrow(tab), "rows\\n")
+    cat("Created table with", nrow(tab), "rows\\n")
     "PASS"
   `);
-
   if (loadResult !== "PASS") {
-    console.error("Package load test FAILED");
-    await webR.close();
-    server.close();
-    process.exit(1);
+    throw new Error("Smoke test failed");
   }
-  console.log("✓ Package loads and works correctly");
-
-  // Run tests using test_dir() since the wasm binary doesn't include tests
-  console.log("Running testthat suite under webR...");
+  console.log("✓ Smoke test passed");
 
+  // Run testthat suite
+  console.log("Running testthat suite...");
   const testResult = await webR.evalRString(`
     library(testthat)
-    library(arrow)
-    results <- testthat::test_dir("${vfsTestDir}", reporter = "summary", stop_on_failure = FALSE, package = "arrow")
+    results <- testthat::test_dir(
+      "${vfsTestDir}",
+      reporter = "summary",
+      stop_on_failure = FALSE,
+      package = "arrow"
+    )
     df <- as.data.frame(results)
-    n_pass <- sum(df$passed)
-    n_skip <- sum(df$skipped)
-    n_fail <- sum(df$failed)
-    n_error <- sum(df$error)
     cat(sprintf("Results: %d passed, %d skipped, %d failed, %d errors\\n",
-                n_pass, n_skip, n_fail, n_error))
-    if (n_fail > 0 || n_error > 0) "FAIL" else "PASS"
+                sum(df$passed), sum(df$skipped), sum(df$failed), sum(df$error)))
+    if (sum(df$failed) > 0 || sum(df$error) > 0) "FAIL" else "PASS"
   `);
-
   if (testResult !== "PASS") {
-    console.error("testthat suite FAILED");
-    await webR.close();
-    server.close();
-    process.exit(1);
+    throw new Error("testthat suite failed");
   }
   console.log("✓ testthat suite passed");
 
-  console.log("✓ All tests passed!");
   await webR.close();
   server.close();
 }

From 2ef885c1e6ab6cb408bfc65f960e81f8902742bb Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 20 May 2026 21:01:06 -0700
Subject: [PATCH 08/22] configure tzdb

---
 r/R/arrow-package.R | 2 ++
 1 file changed, 2 insertions(+)

diff --git a/r/R/arrow-package.R b/r/R/arrow-package.R
index 721165ab114e..8b2c57c9184d 100644
--- a/r/R/arrow-package.R
+++ b/r/R/arrow-package.R
@@ -153,6 +153,8 @@ s3_finalizer <- new.env(parent = emptyenv())
   if (identical(R.version$os, "emscripten")) {
     # Disable multithreading on Wasm/Emscripten
     options(arrow.use_threads = FALSE)
+    # No system tzdata on Emscripten; use the tzdb R package
+    configure_tzdb()
   }
 
   if (tolower(Sys.info()[["sysname"]]) == "windows") {

From b0d79f1dfc99fd1f3addd2412c529614ddebef5a Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 20 May 2026 21:05:28 -0700
Subject: [PATCH 09/22] add skip

---
 r/tests/testthat/helper-skip.R       | 4 ++++
 r/tests/testthat/test-dplyr-filter.R | 2 ++
 2 files changed, 6 insertions(+)

diff --git a/r/tests/testthat/helper-skip.R b/r/tests/testthat/helper-skip.R
index 133b03798813..283d5578d7dd 100644
--- a/r/tests/testthat/helper-skip.R
+++ b/r/tests/testthat/helper-skip.R
@@ -101,6 +101,10 @@ skip_on_linux_devel <- function() {
   }
 }
 
+skip_on_emscripten <- function() {
+  skip_if(identical(R.version$os, "emscripten"), "Not supported on Emscripten")
+}
+
 skip_on_r_older_than <- function(r_version) {
   if (force_tests()) {
     return()
diff --git a/r/tests/testthat/test-dplyr-filter.R b/r/tests/testthat/test-dplyr-filter.R
index ad69b26be798..bc912c343c0b 100644
--- a/r/tests/testthat/test-dplyr-filter.R
+++ b/r/tests/testthat/test-dplyr-filter.R
@@ -415,6 +415,8 @@ test_that("filter() with namespaced functions", {
 })
 
 test_that("filter() with across()", {
+  skip_on_emscripten() # TODO(xxx): need to figure out what warnings this throws
+
   compare_dplyr_binding(
     .input |>
       filter(if_any(ends_with("l"), ~ is.na(.))) |>

From b9c6df0bd4821a83126d8681d184fd83d32dade2 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Thu, 21 May 2026 08:00:35 -0700
Subject: [PATCH 10/22] add debug info for ci

---
 r/R/arrow-package.R | 22 ++++++++++++++++++----
 1 file changed, 18 insertions(+), 4 deletions(-)

diff --git a/r/R/arrow-package.R b/r/R/arrow-package.R
index 8b2c57c9184d..20514f704566 100644
--- a/r/R/arrow-package.R
+++ b/r/R/arrow-package.R
@@ -190,19 +190,33 @@ configure_tzdb <- function() {
     tryCatch(
       {
         tzdb::tzdb_initialize()
-        set_timezone_database(tzdb::tzdb_path("text"))
+        tz_path <- tzdb::tzdb_path("text")
+        packageStartupMessage("[configure_tzdb] tzdb path: ", tz_path)
+        packageStartupMessage("[configure_tzdb] path exists: ", dir.exists(tz_path))
+        if (dir.exists(tz_path)) {
+          tz_files <- list.files(tz_path, recursive = TRUE)
+          packageStartupMessage(
+            "[configure_tzdb] tzdb contents (", length(tz_files), " files): ",
+            paste(head(tz_files, 10), collapse = ", "),
+            if (length(tz_files) > 10) "..."
+          )
+        }
+        set_timezone_database(tz_path)
+        packageStartupMessage("[configure_tzdb] successfully configured timezone database")
       },
       error = function(e) {
         packageStartupMessage(
-          "The tzdb package was available but failed to initialize: ",
-          e,
+          "[configure_tzdb] tzdb package available but failed to initialize: ",
+          conditionMessage(e)
+        )
+        packageStartupMessage(
           "Timezones will not be available to Arrow compute functions."
         )
       }
     )
   } else {
     packageStartupMessage(
-      "The tzdb package is not installed. ",
+      "[configure_tzdb] tzdb package is NOT installed. ",
       "Timezones will not be available to Arrow compute functions. ",
       "If you get errors when using Arrow on datetimes, try running ",
       "`install.packages('tzdb')` and trying again."

From 88ab8418c84c5355a47662dd6de5018c4447e0ce Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 17:41:35 -0700
Subject: [PATCH 11/22] add more skips

---
 r/tests/testthat/test-dataset-dplyr.R | 2 ++
 r/tests/testthat/test-dplyr-arrange.R | 1 +
 r/tests/testthat/test-dplyr-query.R   | 1 +
 3 files changed, 4 insertions(+)

diff --git a/r/tests/testthat/test-dataset-dplyr.R b/r/tests/testthat/test-dataset-dplyr.R
index 09ca0ef155b7..ff6fe435747a 100644
--- a/r/tests/testthat/test-dataset-dplyr.R
+++ b/r/tests/testthat/test-dataset-dplyr.R
@@ -70,6 +70,7 @@ test_that("filter() with %in%", {
 })
 
 test_that("filter() on timestamp columns", {
+  skip_on_emscripten()
   skip_if_not_available("re2")
 
   ds <- open_dataset(dataset_dir, partitioning = schema(part = uint8()))
@@ -119,6 +120,7 @@ test_that("filter() on date32 columns", {
   )
 
   skip_if_not_available("re2")
+  skip_on_emscripten()
 
   # Also with timestamp scalar
   expect_equal(
diff --git a/r/tests/testthat/test-dplyr-arrange.R b/r/tests/testthat/test-dplyr-arrange.R
index d502a7f040a1..3d0dd5066654 100644
--- a/r/tests/testthat/test-dplyr-arrange.R
+++ b/r/tests/testthat/test-dplyr-arrange.R
@@ -238,6 +238,7 @@ test_that("Can use across() within arrange()", {
       collect(),
     example_data
   )
+  skip_on_emscripten()
   compare_dplyr_binding(
     .input |>
       arrange(across(starts_with("d"), desc)) |>
diff --git a/r/tests/testthat/test-dplyr-query.R b/r/tests/testthat/test-dplyr-query.R
index 53864a166337..90a502e5428e 100644
--- a/r/tests/testthat/test-dplyr-query.R
+++ b/r/tests/testthat/test-dplyr-query.R
@@ -646,6 +646,7 @@ test_that("Scalars in expressions match the type of the field, if possible", {
   # it merges.
   # https://github.com/r-hub/rhub-linux-builders/pull/65
   skip_if(identical(Sys.timezone(), "/UTC"))
+  skip_on_emscripten()
 
   expect_output(
     tab |>

From d29b200692c7e2e5cb38df6abfbd12ed4b620b81 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 17:42:40 -0700
Subject: [PATCH 12/22] Format arrow-package.R with Air

---
 r/R/arrow-package.R | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/r/R/arrow-package.R b/r/R/arrow-package.R
index 20514f704566..d8e443d10046 100644
--- a/r/R/arrow-package.R
+++ b/r/R/arrow-package.R
@@ -196,7 +196,9 @@ configure_tzdb <- function() {
         if (dir.exists(tz_path)) {
           tz_files <- list.files(tz_path, recursive = TRUE)
           packageStartupMessage(
-            "[configure_tzdb] tzdb contents (", length(tz_files), " files): ",
+            "[configure_tzdb] tzdb contents (",
+            length(tz_files),
+            " files): ",
             paste(head(tz_files, 10), collapse = ", "),
             if (length(tz_files) > 10) "..."
           )

From e53e9475772e5f1567900811bb0df5cd274858cd Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 19:07:04 -0700
Subject: [PATCH 13/22] Fix gmake invocation failure

---
 r/inst/build_arrow_static.sh | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/r/inst/build_arrow_static.sh b/r/inst/build_arrow_static.sh
index 349531b75fd9..0df5240888ab 100755
--- a/r/inst/build_arrow_static.sh
+++ b/r/inst/build_arrow_static.sh
@@ -114,7 +114,7 @@ ${CMAKE_WRAPPER} ${CMAKE} -DARROW_BOOST_USE_SHARED=OFF \
     -G "${CMAKE_GENERATOR:-Unix Makefiles}" \
     ${SOURCE_DIR}
 
-${CMAKE} --build . --target install -- -j $N_JOBS
+${CMAKE} --build . --target install --parallel $N_JOBS
 
 if command -v sccache &> /dev/null; then
   echo "=== sccache stats after the build ==="

From 89d08692ac45cb1de626a96e9b90429755ee0389 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 19:29:17 -0700
Subject: [PATCH 14/22] Update build_arrow_static.sh

---
 r/inst/build_arrow_static.sh | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/r/inst/build_arrow_static.sh b/r/inst/build_arrow_static.sh
index 0df5240888ab..26b11453eda2 100755
--- a/r/inst/build_arrow_static.sh
+++ b/r/inst/build_arrow_static.sh
@@ -38,9 +38,9 @@ DEST_DIR="$(mkdir -p "${DEST_DIR}" && cd "${DEST_DIR}" && pwd)"
 
 if [ "$N_JOBS" = "" ]; then
   if [ "`uname -s`" = "Darwin" ]; then
-    N_JOBS="$(sysctl -n hw.logicalcpu)"
+    N_JOBS="$(sysctl -n hw.logicalcpu 2>/dev/null || echo 2)"
   else
-    N_JOBS="$(nproc)"
+    N_JOBS="$(nproc 2>/dev/null || echo 2)"
   fi
 fi
 
@@ -114,7 +114,7 @@ ${CMAKE_WRAPPER} ${CMAKE} -DARROW_BOOST_USE_SHARED=OFF \
     -G "${CMAKE_GENERATOR:-Unix Makefiles}" \
     ${SOURCE_DIR}
 
-${CMAKE} --build . --target install --parallel $N_JOBS
+MAKEFLAGS="-j$N_JOBS" ${CMAKE} --build . --target install
 
 if command -v sccache &> /dev/null; then
   echo "=== sccache stats after the build ==="

From 0eb079b459bb52bc5aa8fad3fb56daf69245c97e Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 19:32:43 -0700
Subject: [PATCH 15/22] Make r_wasm_test.cjs slightly safer

---
 ci/scripts/r_wasm_test.cjs | 5 +++++
 1 file changed, 5 insertions(+)

diff --git a/ci/scripts/r_wasm_test.cjs b/ci/scripts/r_wasm_test.cjs
index 054119039861..ccc6cb393bad 100644
--- a/ci/scripts/r_wasm_test.cjs
+++ b/ci/scripts/r_wasm_test.cjs
@@ -54,6 +54,11 @@ async function main() {
   // Serve the repo over HTTP (webR can't access the host filesystem directly)
   const server = http.createServer((req, res) => {
     const filePath = path.join(repoDir, decodeURIComponent(req.url));
+    if (!filePath.startsWith(path.resolve(repoDir))) {
+      res.writeHead(403);
+      res.end();
+      return;
+    }
     fs.readFile(filePath, (err, data) => {
       if (err) {
         res.writeHead(404);

From 03e9002c8655d2c3b646042613b66da84f3d4fc2 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 19:34:48 -0700
Subject: [PATCH 16/22] respect force_tests in skip_on_emscripten

---
 r/tests/testthat/helper-skip.R | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/r/tests/testthat/helper-skip.R b/r/tests/testthat/helper-skip.R
index 283d5578d7dd..ed2bfae0dbb7 100644
--- a/r/tests/testthat/helper-skip.R
+++ b/r/tests/testthat/helper-skip.R
@@ -102,6 +102,10 @@ skip_on_linux_devel <- function() {
 }
 
 skip_on_emscripten <- function() {
+  if (force_tests()) {
+    return()
+  }
+
   skip_if(identical(R.version$os, "emscripten"), "Not supported on Emscripten")
 }
 

From f309ad9869a95dde72856e9a6d738340b9ecab48 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 19:36:35 -0700
Subject: [PATCH 17/22] Remove TODO

---
 r/tests/testthat/test-dplyr-filter.R | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

diff --git a/r/tests/testthat/test-dplyr-filter.R b/r/tests/testthat/test-dplyr-filter.R
index bc912c343c0b..7b0ca374bd03 100644
--- a/r/tests/testthat/test-dplyr-filter.R
+++ b/r/tests/testthat/test-dplyr-filter.R
@@ -415,7 +415,7 @@ test_that("filter() with namespaced functions", {
 })
 
 test_that("filter() with across()", {
-  skip_on_emscripten() # TODO(xxx): need to figure out what warnings this throws
+  skip_on_emscripten()
 
   compare_dplyr_binding(
     .input |>

From 3866da32e850a3a6e004628d8b6b821b6018e156 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 19:43:16 -0700
Subject: [PATCH 18/22] revert our changes to build_arrow_static.sh

---
 r/inst/build_arrow_static.sh | 6 +++---
 1 file changed, 3 insertions(+), 3 deletions(-)

diff --git a/r/inst/build_arrow_static.sh b/r/inst/build_arrow_static.sh
index 26b11453eda2..349531b75fd9 100755
--- a/r/inst/build_arrow_static.sh
+++ b/r/inst/build_arrow_static.sh
@@ -38,9 +38,9 @@ DEST_DIR="$(mkdir -p "${DEST_DIR}" && cd "${DEST_DIR}" && pwd)"
 
 if [ "$N_JOBS" = "" ]; then
   if [ "`uname -s`" = "Darwin" ]; then
-    N_JOBS="$(sysctl -n hw.logicalcpu 2>/dev/null || echo 2)"
+    N_JOBS="$(sysctl -n hw.logicalcpu)"
   else
-    N_JOBS="$(nproc 2>/dev/null || echo 2)"
+    N_JOBS="$(nproc)"
   fi
 fi
 
@@ -114,7 +114,7 @@ ${CMAKE_WRAPPER} ${CMAKE} -DARROW_BOOST_USE_SHARED=OFF \
     -G "${CMAKE_GENERATOR:-Unix Makefiles}" \
     ${SOURCE_DIR}
 
-MAKEFLAGS="-j$N_JOBS" ${CMAKE} --build . --target install
+${CMAKE} --build . --target install -- -j $N_JOBS
 
 if command -v sccache &> /dev/null; then
   echo "=== sccache stats after the build ==="

From 172c0d4a27bb68a67de4b7ebd8eb378194ce7b1a Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 19:51:50 -0700
Subject: [PATCH 19/22] revert debug messages in arrow-package.R

---
 r/R/arrow-package.R | 25 ++++---------------------
 1 file changed, 4 insertions(+), 21 deletions(-)

diff --git a/r/R/arrow-package.R b/r/R/arrow-package.R
index d8e443d10046..fcb8cc284036 100644
--- a/r/R/arrow-package.R
+++ b/r/R/arrow-package.R
@@ -190,35 +190,18 @@ configure_tzdb <- function() {
     tryCatch(
       {
         tzdb::tzdb_initialize()
-        tz_path <- tzdb::tzdb_path("text")
-        packageStartupMessage("[configure_tzdb] tzdb path: ", tz_path)
-        packageStartupMessage("[configure_tzdb] path exists: ", dir.exists(tz_path))
-        if (dir.exists(tz_path)) {
-          tz_files <- list.files(tz_path, recursive = TRUE)
-          packageStartupMessage(
-            "[configure_tzdb] tzdb contents (",
-            length(tz_files),
-            " files): ",
-            paste(head(tz_files, 10), collapse = ", "),
-            if (length(tz_files) > 10) "..."
-          )
-        }
-        set_timezone_database(tz_path)
-        packageStartupMessage("[configure_tzdb] successfully configured timezone database")
+        set_timezone_database(tzdb::tzdb_path("text"))
       },
       error = function(e) {
         packageStartupMessage(
-          "[configure_tzdb] tzdb package available but failed to initialize: ",
-          conditionMessage(e)
-        )
-        packageStartupMessage(
-          "Timezones will not be available to Arrow compute functions."
+          "The tzdb package was available but failed to initialize: ",
+          e,
         )
       }
     )
   } else {
     packageStartupMessage(
-      "[configure_tzdb] tzdb package is NOT installed. ",
+      "The tzdb package is not installed. ",
       "Timezones will not be available to Arrow compute functions. ",
       "If you get errors when using Arrow on datetimes, try running ",
       "`install.packages('tzdb')` and trying again."

From 4bf56698176f77181f98661d225d62108b053b21 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 20:01:04 -0700
Subject: [PATCH 20/22] try to fix tz issue under wasm

---
 cpp/src/arrow/vendored/datetime/visibility.h | 2 +-
 r/tests/testthat/test-dataset-dplyr.R        | 4 ++--
 r/tests/testthat/test-dplyr-arrange.R        | 2 +-
 r/tests/testthat/test-dplyr-filter.R         | 8 ++++++--
 r/tests/testthat/test-dplyr-query.R          | 2 +-
 5 files changed, 11 insertions(+), 7 deletions(-)

diff --git a/cpp/src/arrow/vendored/datetime/visibility.h b/cpp/src/arrow/vendored/datetime/visibility.h
index 780c00d70bd9..1942d87ffc24 100644
--- a/cpp/src/arrow/vendored/datetime/visibility.h
+++ b/cpp/src/arrow/vendored/datetime/visibility.h
@@ -17,7 +17,7 @@
 
 #pragma once
 
-#ifndef _WIN32
+#if !defined(_WIN32) && !defined(__EMSCRIPTEN__)
 #  define USE_OS_TZDB 1
 #endif
 
diff --git a/r/tests/testthat/test-dataset-dplyr.R b/r/tests/testthat/test-dataset-dplyr.R
index ff6fe435747a..a755ecebbd66 100644
--- a/r/tests/testthat/test-dataset-dplyr.R
+++ b/r/tests/testthat/test-dataset-dplyr.R
@@ -70,7 +70,7 @@ test_that("filter() with %in%", {
 })
 
 test_that("filter() on timestamp columns", {
-  skip_on_emscripten()
+  # skip_on_emscripten()  # TODO: remove if timezone fix (visibility.h) works
   skip_if_not_available("re2")
 
   ds <- open_dataset(dataset_dir, partitioning = schema(part = uint8()))
@@ -120,7 +120,7 @@ test_that("filter() on date32 columns", {
   )
 
   skip_if_not_available("re2")
-  skip_on_emscripten()
+  # skip_on_emscripten()  # TODO: remove if timezone fix (visibility.h) works
 
   # Also with timestamp scalar
   expect_equal(
diff --git a/r/tests/testthat/test-dplyr-arrange.R b/r/tests/testthat/test-dplyr-arrange.R
index 3d0dd5066654..2412bbd2e1e8 100644
--- a/r/tests/testthat/test-dplyr-arrange.R
+++ b/r/tests/testthat/test-dplyr-arrange.R
@@ -238,7 +238,7 @@ test_that("Can use across() within arrange()", {
       collect(),
     example_data
   )
-  skip_on_emscripten()
+  # skip_on_emscripten() # TODO: Unrelated to timezone fix; passing a bare function (desc) to across() .fns fails on Emscripten
   compare_dplyr_binding(
     .input |>
       arrange(across(starts_with("d"), desc)) |>
diff --git a/r/tests/testthat/test-dplyr-filter.R b/r/tests/testthat/test-dplyr-filter.R
index 7b0ca374bd03..fe7c84c80a8d 100644
--- a/r/tests/testthat/test-dplyr-filter.R
+++ b/r/tests/testthat/test-dplyr-filter.R
@@ -25,7 +25,11 @@ tbl <- example_data
 tbl$verses <- verses[[1]]
 # c(" a ", "  b  ", "   c   ", ...) increasing padding
 # nchar =   3  5  7  9 11 13 15 17 19 21
-tbl$padded_strings <- stringr::str_pad(letters[1:10], width = 2 * (1:10) + 1, side = "both")
+tbl$padded_strings <- stringr::str_pad(
+  letters[1:10],
+  width = 2 * (1:10) + 1,
+  side = "both"
+)
 tbl$some_negative <- tbl$int * (-1)^(1:nrow(tbl)) # nolint
 
 test_that("filter() on is.na()", {
@@ -415,7 +419,7 @@ test_that("filter() with namespaced functions", {
 })
 
 test_that("filter() with across()", {
-  skip_on_emscripten()
+  #  skip_on_emscripten() # TODO: Remove if timezone fix works
 
   compare_dplyr_binding(
     .input |>
diff --git a/r/tests/testthat/test-dplyr-query.R b/r/tests/testthat/test-dplyr-query.R
index 90a502e5428e..257e9dabe317 100644
--- a/r/tests/testthat/test-dplyr-query.R
+++ b/r/tests/testthat/test-dplyr-query.R
@@ -646,7 +646,7 @@ test_that("Scalars in expressions match the type of the field, if possible", {
   # it merges.
   # https://github.com/r-hub/rhub-linux-builders/pull/65
   skip_if(identical(Sys.timezone(), "/UTC"))
-  skip_on_emscripten()
+  # skip_on_emscripten()  # TODO: remove if timezone fix (visibility.h) works
 
   expect_output(
     tab |>

From 093a443c2cd29697f30198b1c6b45ed48625df12 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 20:36:06 -0700
Subject: [PATCH 21/22] Fix cmake issue from nixlibs

---
 r/tools/nixlibs.R | 4 ++++
 1 file changed, 4 insertions(+)

diff --git a/r/tools/nixlibs.R b/r/tools/nixlibs.R
index ba705e03ad7e..2a8a7821f1c7 100644
--- a/r/tools/nixlibs.R
+++ b/r/tools/nixlibs.R
@@ -527,6 +527,10 @@ build_libarrow <- function(src_dir, dst_dir) {
   # CRAN policy says not to use more than 2 cores during checks
   # If you have more and want to use more, set MAKEFLAGS or NOT_CRAN
   ncores <- parallel::detectCores()
+  # detectCores() returns NA in some environments under emscripten, handle it
+  if (is.na(ncores)) {
+    ncores <- 1L
+  }
   if (!not_cran) {
     ncores <- min(ncores, 2)
   }

From 9cc126437bdee30b0bba036ce7d31bb09e449801 Mon Sep 17 00:00:00 2001
From: Bryce Mecum <petridish@gmail.com>
Date: Wed, 1 Jul 2026 23:38:13 -0700
Subject: [PATCH 22/22] Syntax fix

---
 build-wasm.log                               |  5 +++
 cpp/src/arrow/vendored/datetime/visibility.h |  8 ++++-
 r/R/arrow-package.R                          |  2 +-
 r/run-wasm-build-and-test.sh                 | 33 ++++++++++++++++++++
 4 files changed, 46 insertions(+), 2 deletions(-)
 create mode 100644 build-wasm.log
 create mode 100644 r/run-wasm-build-and-test.sh

diff --git a/build-wasm.log b/build-wasm.log
new file mode 100644
index 000000000000..a409358a6be3
--- /dev/null
+++ b/build-wasm.log
@@ -0,0 +1,5 @@
+> if (!requireNamespace("pak", quietly = TRUE)) install.packages("pak", repos = "https://cloud.r-project.org"); if (!requireNamespace("rwasm", quietly = TRUE)) pak::pak("r-wasm/rwasm"); rwasm::build(".")
+Error in if (startsWith(url, "file:")) "internal" else "libcurl" : 
+  missing value where TRUE/FALSE needed
+Calls: <Anonymous> -> make_remote_tarball -> download.file
+Execution halted
diff --git a/cpp/src/arrow/vendored/datetime/visibility.h b/cpp/src/arrow/vendored/datetime/visibility.h
index 1942d87ffc24..492d189a0397 100644
--- a/cpp/src/arrow/vendored/datetime/visibility.h
+++ b/cpp/src/arrow/vendored/datetime/visibility.h
@@ -17,10 +17,16 @@
 
 #pragma once
 
-#if !defined(_WIN32) && !defined(__EMSCRIPTEN__)
+#ifndef _WIN32
 #  define USE_OS_TZDB 1
 #endif
 
+#ifdef __EMSCRIPTEN__
+// Emscripten has no OS timezone database and no curl, so disable both.
+#  undef USE_OS_TZDB
+#  define HAS_REMOTE_API 0
+#endif
+
 #if defined(ARROW_STATIC)
 // intentially empty
 #elif defined(ARROW_EXPORTING)
diff --git a/r/R/arrow-package.R b/r/R/arrow-package.R
index fcb8cc284036..179bc1945c30 100644
--- a/r/R/arrow-package.R
+++ b/r/R/arrow-package.R
@@ -195,7 +195,7 @@ configure_tzdb <- function() {
       error = function(e) {
         packageStartupMessage(
           "The tzdb package was available but failed to initialize: ",
-          e,
+          e
         )
       }
     )
diff --git a/r/run-wasm-build-and-test.sh b/r/run-wasm-build-and-test.sh
new file mode 100644
index 000000000000..d89530b310c8
--- /dev/null
+++ b/r/run-wasm-build-and-test.sh
@@ -0,0 +1,33 @@
+#!/usr/bin/env bash
+# Build and test the Arrow R package for WebAssembly locally,
+# replicating the test-r-wasm crossbow job exactly.
+#
+# Usage: bash r/run-wasm-build-and-test.sh
+# Run from the repo root.
+
+set -euxo pipefail
+
+cd "$(git rev-parse --show-toplevel)/r"
+
+# Step 1: sync C++ source and build R source tarball (done on host, not in Docker)
+make sync-cpp
+R CMD build --no-build-vignettes .
+
+# Step 2: build Wasm binary inside the R-universe container
+docker run --rm \
+  -v "${PWD}:/work" \
+  -w /work \
+  ghcr.io/r-universe-org/build-wasm:latest \
+  bash -lc '
+    set -euxo pipefail
+    R -q -e "if (!requireNamespace(\"pak\", quietly = TRUE)) install.packages(\"pak\", repos = \"https://cloud.r-project.org\"); if (!requireNamespace(\"rwasm\", quietly = TRUE)) pak::pak(\"r-wasm/rwasm\"); rwasm::build(\".\")" \
+      2>&1 | tee build-wasm.log
+  '
+
+# Step 3: run the test suite inside the R-universe container
+ARROW_ROOT="$(git rev-parse --show-toplevel)"
+docker run --rm \
+  -v "${ARROW_ROOT}:/arrow" \
+  -w /tmp \
+  ghcr.io/r-universe-org/build-wasm:latest \
+  bash /arrow/ci/scripts/r_wasm_test.sh /arrow/r