Make ShowIncludesFilter compatible with the sibling repository layout…

… by grabbing everything under the execroot base and prepend the collected paths with "..\".

PiperOrigin-RevId: 356735700

src/main/java/com/google/devtools/build/lib/rules/cpp/ShowIncludesFilter.java

@@ -148,8 +148,10 @@ public static class FilterShowIncludesOutputStream extends FilterOutputStream {
                 StandardCharsets.UTF_8) // Spanish
             );
     private final String sourceFileName;
-    private static final Pattern EXECROOT_HEADER_PATTERN =
-        Pattern.compile(".*execroot\\\\[^\\\\]*\\\\(?<headerPath>.*)");
+    // Grab everything under the execroot base so that external repository header files are covered
+    // in the sibling repository layout.
+    private static final Pattern EXECROOT_BASE_HEADER_PATTERN =
+        Pattern.compile(".*execroot\\\\(?<headerPath>.*)");
 
     public FilterShowIncludesOutputStream(OutputStream out, String sourceFileName) {
       super(out);
@@ -165,9 +167,13 @@ public void write(int b) throws IOException {
         for (String prefix : SHOW_INCLUDES_PREFIXES) {
           if (line.startsWith(prefix)) {
             line = line.substring(prefix.length()).trim();
-            Matcher m = EXECROOT_HEADER_PATTERN.matcher(line);
+            Matcher m = EXECROOT_BASE_HEADER_PATTERN.matcher(line);
             if (m.matches()) {
-              line = m.group("headerPath");
+              // Prefix the matched header path with "..\". This way, external repo header paths are
+              // resolved to "<execroot>\..\<repo name>\<path>", and main repo file paths are
+              // resolved to "<execroot>\..\<main repo>\<path>", which is nicely normalized to
+              // "<execroot>\<path>".
+              line = "..\\" + m.group("headerPath");
             }
             dependencies.add(line);
             prefixMatched = true;

src/test/java/com/google/devtools/build/lib/rules/cpp/ShowIncludesFilterTest.java

@@ -95,7 +95,7 @@ public void testMatchAllOfNotePrefix() throws IOException {
 
   @Test
   // Regression tests for https://github.com/bazelbuild/bazel/issues/9172
-  public void testFindHeaderFromAbsolutePathUnderExecroot() throws IOException {
+  public void testFindHeaderFromAbsolutePathUnderExecrootBase() throws IOException {
     // "Note: including file:" is the prefix
     filterOutputStream.write(
         getBytes("Note: including file: C:\\tmp\\xxxx\\execroot\\__main__\\foo\\bar\\bar.h"));
@@ -105,7 +105,7 @@ public void testFindHeaderFromAbsolutePathUnderExecroot() throws IOException {
     filterOutputStream.write(getBytes("\n"));
     // It's a match, output should be filtered, dependency on bar.h should be found.
     assertThat(output.toString()).isEmpty();
-    assertThat(showIncludesFilter.getDependencies()).contains("foo\\bar\\bar.h");
+    assertThat(showIncludesFilter.getDependencies()).contains("..\\__main__\\foo\\bar\\bar.h");
   }
 
   @Test