plugin: Add test for streaming output handler

Author: euanh

.github/workflows/integration_tests.yml

@@ -34,3 +34,25 @@ jobs:
                   REGISTRY_PORT: 5000
               run: |
                   swift test
+
+    plugin-streaming-output-test:
+        name: Plugin streaming output test
+        runs-on: ubuntu-latest
+        services:
+            registry:
+                image: registry:2
+                ports:
+                    - 5000:5000
+        steps:
+            - name: Checkout repository
+              uses: actions/checkout@v4
+              with:
+                  persist-credentials: false
+
+            - name: Mark the workspace as safe
+              # https://github.com/actions/checkout/issues/766
+              run: git config --global --add safe.directory ${GITHUB_WORKSPACE}
+
+            - name: Check plugin streaming output is reassembled and printed properly
+              run: |
+                  scripts/test-plugin-output-streaming.sh

scripts/test-plugin-output-streaming.sh

@@ -0,0 +1,38 @@
+#!/usr/bin/env bash
+
+# Test that error output streamed from containertool is printed correctly by the plugin. 
+
+set -exo pipefail
+
+log() { printf -- "** %s\n" "$*" >&2; }
+error() { printf -- "** ERROR: %s\n" "$*" >&2; }
+fatal() { error "$@"; exit 1; }
+
+# Work in a temporary directory, deleted after the test finishes
+PKGPATH=$(mktemp -d)
+cleanup() {
+  log "Deleting temporary package $PKGPATH"
+  rm -rf "$PKGPATH"
+}
+trap cleanup EXIT
+
+swift package --package-path "$PKGPATH" init --type executable --name hello
+cat >> "$PKGPATH/Package.swift" <<EOF
+package.dependencies += [
+    .package(path: "$PWD"),
+]
+EOF
+
+# Run the plugin, forgetting a mandatory argument.   Verify that the output is not corrupted.
+# The `swift package` command will return a nonzero exit code.   This is expected, so disable pipefail.
+set +o pipefail
+swift package --package-path "$PKGPATH" --allow-network-connections all build-container-image 2>&1 | tee "$PKGPATH/output"
+set -o pipefail
+
+grep -F -x -e "error: Missing expected argument '--repository <repository>'" \
+           -e "error: Help:  --repository <repository>  Repository path" \
+           -e "error: Usage: containertool [<options>] --repository <repository> <executable>" \
+           -e "error:   See 'containertool --help' for more information." "$PKGPATH/output"
+
+echo Plugin error output: PASSED
+