ci(swift): isolate SwiftPM cache per runner to end the prove-swift race

Two self-hosted macOS runners (tsavo-mac-x64 + -2) run under one user on
one host, so they share SwiftPM's global cache (~/Library/Caches/
org.swift.swiftpm). When two swift PRs build concurrently — one per runner —
both 'swift build' processes write that shared cache and race, failing the
prove-swift (macOS) C1-C8 job. Evidence: #1536 passed prove-swift solo; #1537
and #1539 both failed it running concurrently. The .build scratch dir is
already per-checkout; only the shared --cache-path was contended.

Thread an opt-in SWIFT_CACHE_PATH through the Makefile (build-swift +
test-swift) to 'swift build/test --cache-path', and set it to
${{ runner.temp }}/swiftpm-cache on both macOS swift jobs. RUNNER_TEMP is
per-runner-instance, so concurrent builds get distinct caches; sequential
builds on one runner still reuse it. Unset (local dev) keeps SwiftPM's
default — verified behavior-identical via 'make -n build-swift'.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

Author: TSavo

.github/workflows/ci.yml

@@ -328,6 +328,13 @@ jobs:
     # (swift, rust) are pre-installed on the runner host.
     runs-on: [self-hosted, macOS, X64]
     timeout-minutes: 45
+    env:
+      # Isolate the SwiftPM shared cache (--cache-path) per runner instance so
+      # two macOS runners on one host don't race on the shared
+      # ~/Library/Caches/org.swift.swiftpm — the prove-swift flake. RUNNER_TEMP
+      # is per-runner, so concurrent swift builds get distinct caches; .build
+      # (compiled artifacts) is still restored from the GH cache below.
+      SWIFT_CACHE_PATH: ${{ runner.temp }}/swiftpm-cache
 
     steps:
       - name: Checkout
@@ -612,6 +619,12 @@ jobs:
     runs-on: [self-hosted, macOS, X64]
     timeout-minutes: 20
     needs: conformance
+    env:
+      # Per-runner SwiftPM cache isolation — see the conformance-macos-swift
+      # job above. Two macOS runners on one host raced on the shared SwiftPM
+      # cache (~/Library/Caches/org.swift.swiftpm), failing concurrent
+      # prove-swift runs; RUNNER_TEMP gives each a distinct --cache-path.
+      SWIFT_CACHE_PATH: ${{ runner.temp }}/swiftpm-cache
 
     steps:
       - name: Checkout

Makefile

@@ -288,8 +288,16 @@ build-ruby:
 # runtime perf is irrelevant. Override with `SWIFT_BUILD_CONFIG=release` if a
 # release artifact is ever needed.
 SWIFT_BUILD_CONFIG ?= debug
+# Per-invocation SwiftPM cache isolation. SwiftPM's *scratch* dir (`.build`)
+# is already per-checkout, but its *shared* cache (--cache-path, default
+# ~/Library/Caches/org.swift.swiftpm) is per-user. On a host running two
+# self-hosted runners under one account, two concurrent `swift build`s race
+# on that shared cache and both fail (the prove-swift macOS flake). Set
+# SWIFT_CACHE_PATH to a per-runner path (e.g. $RUNNER_TEMP/swiftpm-cache) to
+# isolate it. Unset (local dev) keeps SwiftPM's default — behavior unchanged.
+SWIFT_CACHE_ARG := $(if $(SWIFT_CACHE_PATH),--cache-path $(SWIFT_CACHE_PATH),)
 build-swift:
-	cd implementations/swift && swift build -c $(SWIFT_BUILD_CONFIG)
+	cd implementations/swift && swift build -c $(SWIFT_BUILD_CONFIG) $(SWIFT_CACHE_ARG)
 
 # --- Mint targets ------------------------------------------------------------
 
@@ -808,7 +816,7 @@ test-java: build-java
 
 .PHONY: test-swift
 test-swift: build-swift
-	cd implementations/swift && swift test
+	cd implementations/swift && swift test $(SWIFT_CACHE_ARG)
 	cd implementations/swift && swift run conformance
 	cd implementations/swift && swift run test-swift-lsp
 	cd implementations/swift && swift run test-swift-crypto