Resolve external paths to their workspaces

This prevents header include path breakage when the execroot is reconfigured on rebuild.
See bazelbuild#2766

base/src/com/google/idea/blaze/base/sync/workspace/ExecutionRootPathResolver.java

@@ -31,8 +31,9 @@
  * Converts execution-root-relative paths to absolute files with a minimum of file system calls
  * (typically none).
  *
- * <p>Files which exist both underneath the execution root and within the workspace will be resolved
- * to workspace paths.
+ * <p>Files which exist both underneath the execution root and within a workspace will be resolved
+ * to paths within their workspace. This prevents those paths from being broken when a different
+ * target is built.
  */
 public class ExecutionRootPathResolver {
 
@@ -76,13 +77,13 @@ public File resolveExecutionRootPath(ExecutionRootPath path) {
     if (isInWorkspace(path)) {
       return workspacePathResolver.resolveToFile(path.getAbsoluteOrRelativeFile().getPath());
     }
-    return path.getFileRootedAt(executionRoot);
+    return convertExternalToStable(path.getFileRootedAt(executionRoot));
   }
 
   /**
    * This method should be used for directories. Returns all workspace files corresponding to the
-   * given execution-root-relative path. If the file does not exist inside the workspace (e.g. for
-   * blaze output files or external workspace files), returns the path rooted in the execution root.
+   * given execution-root-relative path. If the file does not exist inside a workspace (e.g. for
+   * blaze output files), returns the path rooted in the execution root.
    */
   public ImmutableList<File> resolveToIncludeDirectories(ExecutionRootPath path) {
     if (path.isAbsolute()) {
@@ -97,7 +98,7 @@ public ImmutableList<File> resolveToIncludeDirectories(ExecutionRootPath path) {
         return ImmutableList.of();
       }
     }
-    return ImmutableList.of(path.getFileRootedAt(executionRoot));
+    return ImmutableList.of(convertExternalToStable(path.getFileRootedAt(executionRoot)));
   }
 
   public File getExecutionRoot() {
@@ -118,4 +119,19 @@ private static String getFirstPathComponent(String path) {
   private static boolean isExternalWorkspacePath(String firstPathComponent) {
     return firstPathComponent.equals("external");
   }
+
+  /**
+   * Converts external paths in unstable locations under execroot that may change on the next build
+   * to stable ones under their external workspace.
+   * That is, converts paths under <outputBase>/execroot/__main__/external/ to paths under
+   * <outputBase>/external/.
+   * Safe to call on all paths; non-external paths are left as they are.
+   * See https://github.com/bazelbuild/intellij/issues/2766 for more details.
+   */
+  private static File convertExternalToStable(File f) {
+    String unstableExecrootSubpath =
+      f.separator + "execroot" + f.separator + "__main__" + f.separator + "external" + f.separator;
+    String stableExternalSubpath = f.separator + "external" + f.separator;
+    return new File(f.getAbsolutePath().replace(unstableExecrootSubpath, stableExternalSubpath));
+  }
 }