fix(downloader): handle invalid regex in downloader config

[Ankush1oo8]

Description:

This pull request addresses Issue #20832, where an invalid regex in the --experimental_downloader_config file caused Bazel to crash silently without an error message.

Changes Introduced:

• Added a try-catch block to catch PatternSyntaxException when compiling regex patterns in UrlRewriterConfig.java.
• Throws a UrlRewriterParseException with a clear error message instead of crashing.
• Includes details on the regex error, such as the invalid sequence and its index in the pattern.

Before the Fix:

• An invalid regex in downloader.cfg (e.g., rewrite (.*\maven.org/.*) http://example.com/mirror/$1) caused Bazel to crash with error code 37, but no explanation was given.

After the Fix:

• Users now see a clear error message pointing out the invalid regex and its exact issue:

  Invalid regex in `rewrite`: Illegal/unsupported escape sequence at index 4 in `(.*\maven.org/.*)`
  

• This helps users quickly debug and fix their configuration.

Testing & Verification:

• Manually tested by providing valid and invalid regex patterns in downloader.cfg.
• Verified that valid patterns work correctly and invalid patterns produce a proper error message.

Impact & Notes:

• No breaking changes.
• Improves developer experience by preventing silent crashes and providing actionable feedback.

Related Issue:

Closes #20832

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[sgowroji]

Hi @Ankush1oo8, Could you please take a  look at the above failure

[Ankush1oo8]

@sgowroji Can you help me in what should I do

[sgowroji]

Check the error logs from the failure Buildkite logs https://buildkite.com/bazel/bazel-bazel-github-presubmit/builds/26162#01953eb4-3448-4b5e-87d1-5586237673e7

'''src/main/java/com/google/devtools/build/lib/bazel/repository/downloader/UrlRewriterConfig.java:134: error: cannot find symbol
} catch (PatternSyntaxException e) {
^
symbol: class PatternSyntaxException
location: class UrlRewriterConfig
(20:04:00) INFO: Elapsed time: 18.833s, Critical Path: 6.93s
(20:04:00) INFO: 7723 processes: 6889 remote cache hit, 824 internal, 8 linux-sandbox, 2 local.
(20:04:00) ERROR: Build did NOT complete successfully'''

[Ankush1oo8]

Check the error logs from the failure Buildkite logs https://buildkite.com/bazel/bazel-bazel-github-presubmit/builds/26162#01953eb4-3448-4b5e-87d1-5586237673e7

'''src/main/java/com/google/devtools/build/lib/bazel/repository/downloader/UrlRewriterConfig.java:134: error: cannot find symbol } catch (PatternSyntaxException e) { ^ symbol: class PatternSyntaxException location: class UrlRewriterConfig (20:04:00) INFO: Elapsed time: 18.833s, Critical Path: 6.93s (20:04:00) INFO: 7723 processes: 6889 remote cache hit, 824 internal, 8 linux-sandbox, 2 local. (20:04:00) ERROR: Build did NOT complete successfully'''

Can you check now