build(ci): fix OOM errors on macOS

...by taking physical memory into account when deciding how many test
processes to spawn.

Even though `macos-14` has 3 CPUs, there is only 7GB of memory.
Each test process has a max heap of 2GB, with the addition of
Gradle this is enough to OOM the machine.

So we limit the number of test processes based on RAM

Now:

* 3GB reserved for Gradle (org.gradle.jvmargs=-Xmx3072M) and the OS
* 4GB remaining
* Heap size: 2GB
* => 2 forks

Fixes 21168

Assisted-by: Claude Opus 4.8 - diagnosis & some code

Author: david-allison

build.gradle.kts

@@ -44,6 +44,12 @@ val testSummaryService = System.getenv("GITHUB_STEP_SUMMARY")?.let { path ->
     }
 }
 
+/**
+ * Per-fork JVM heap for unit tests.
+ * See [Test.setMaxHeapSize]
+ */
+val unitTestForkMaxHeapGb = 2
+
 // Here we extract per-module "best practices" settings to a single top-level evaluation
 subprojects {
     apply(plugin = "org.jlleitschuh.gradle.ktlint")
@@ -61,7 +67,7 @@ subprojects {
                 // tell backend to avoid rollover time, and disable interval fuzzing
                 it.environment("ANKI_TEST_MODE", "1")
 
-                it.maxHeapSize = "2g"
+                it.maxHeapSize = "${unitTestForkMaxHeapGb}g"
                 it.minHeapSize = "1g"
 
                 it.useJUnitPlatform()
@@ -158,6 +164,7 @@ if (jvmVersion !in jvmVersionLowerBound..jvmVersionUpperBound) {
 }
 
 val ciBuild by extra(System.getenv("CI") == "true") // true when running on GitHub Actions
+val isMacOs = System.getProperty("os.name") == "Mac OS X"
 // allows for -Dpre-dex=false to be set
 val preDexEnabled by extra("true" == System.getProperty("pre-dex", "true"))
 // allows for universal APKs to be generated
@@ -168,12 +175,26 @@ var androidTestVariantName by extra(
     if (testReleaseBuild) "Release" else "Debug"
 )
 
+private fun sysctl(key: String): Long =
+    providers.exec {
+        commandLine("sysctl", "-n", key)
+    }.standardOutput.asText.get().trim().toLong()
+
 val gradleTestMaxParallelForks by extra(
-    if (System.getProperty("os.name") == "Mac OS X") {
-        // macOS reports hardware cores. This is accurate for CI, Intel (halved due to SMT) and Apple Silicon
-        providers.exec {
-            commandLine("sysctl", "-n", "hw.physicalcpu")
-        }.standardOutput.asText.get().trim().toInt()
+    if (isMacOs) {
+        // macOS reports hardware cores.
+        // This is accurate for CI, Intel (halved due to SMT) and Apple Silicon
+        val physicalCpus = sysctl("hw.physicalcpu")
+
+        if (ciBuild) {
+            // #21168: The `macos-14` CI runner has only 7GB RAM and OOMs (exit 134) so bound by RAM
+            val totalRamGb = sysctl("hw.memsize") / (1024 * 1024 * 1024)
+            val reservedMemory = 3 // 3GB (org.gradle.jvmargs) + OS
+            val memoryBoundForks = max(1L, (totalRamGb - reservedMemory) / unitTestForkMaxHeapGb)
+            minOf(physicalCpus, memoryBoundForks).toInt()
+        } else {
+            physicalCpus.toInt()
+        }
     } else if (ciBuild) {
         // GitHub Actions run on Standard_D4ads_v5 Azure Compute Units with 4 vCPUs
         // They appear to be 2:1 vCPU to CPU on Linux/Windows with two vCPU cores but with performance 1:1-similar