Be more precise about where to read the URL rewriter config from

There is an [open issue](#22104) where bazel will occasionally fail at start up time because it is unable to parse the downloader config. While I've yet to create a reliable reproduction for this issue, the one time I've triggered this error with a build with additional logging, it seemed to be because the current working directory wasn't the root of the workspace. While I'm not sure this will actually fully resolve the problem (since I don't know what's causing it) being clearer about where the config file is meant to be can only be a Good Thing.

Addresses #22104

Closes #22770.

PiperOrigin-RevId: 645115556
Change-Id: I7e9bea1c55a106cd767ebfd6d557a7c59c47cd66

src/main/java/com/google/devtools/build/lib/bazel/BazelRepositoryModule.java

@@ -389,7 +389,8 @@ public void beforeCommand(CommandEnvironment env) throws AbruptExitException {
       }
       try {
         UrlRewriter rewriter =
-            UrlRewriter.getDownloaderUrlRewriter(repoOptions.downloaderConfig, env.getReporter());
+            UrlRewriter.getDownloaderUrlRewriter(
+                env.getWorkspace(), repoOptions.downloaderConfig, env.getReporter());
         downloadManager.setUrlRewriter(rewriter);
       } catch (UrlRewriterParseException e) {
         // It's important that the build stops ASAP, because this config file may be required for

src/main/java/com/google/devtools/build/lib/bazel/repository/downloader/UrlRewriter.java

@@ -41,7 +41,6 @@
 import java.net.URISyntaxException;
 import java.net.URL;
 import java.nio.file.Files;
-import java.nio.file.Paths;
 import java.util.Base64;
 import java.util.Collection;
 import java.util.HashMap;
@@ -85,16 +84,26 @@ public class UrlRewriter {
    * @param configPath Path to the config file to use. May be null.
    * @param reporter Used for logging when URLs are rewritten.
    */
-  public static UrlRewriter getDownloaderUrlRewriter(String configPath, Reporter reporter)
-      throws UrlRewriterParseException {
+  public static UrlRewriter getDownloaderUrlRewriter(
+      Path workspaceRoot, String configPath, Reporter reporter) throws UrlRewriterParseException {
     Consumer<String> log = str -> reporter.handle(Event.info(str));
 
     // "empty" UrlRewriter shouldn't alter auth headers
     if (Strings.isNullOrEmpty(configPath)) {
       return new UrlRewriter(log, "", new StringReader(""));
     }
 
-    try (BufferedReader reader = Files.newBufferedReader(Paths.get(configPath))) {
+    // There have been reports (eg. https://github.com/bazelbuild/bazel/issues/22104) that
+    // there are occasional errors when `configFile` can't be found, and when this happens
+    // investigation suggests that the current working directory isn't the workspace root.
+    Path actualConfigPath = workspaceRoot.getRelative(configPath);
+
+    if (!actualConfigPath.exists()) {
+      throw new UrlRewriterParseException(
+          String.format("Unable to find downloader config file %s", configPath));
+    }
+
+    try (BufferedReader reader = Files.newBufferedReader(actualConfigPath.getPathFile().toPath())) {
       return new UrlRewriter(log, configPath, reader);
     } catch (IOException e) {
       throw new UrlRewriterParseException(e.getMessage());