[8.6.0] Compensate for Windows filesystems lacking junction support#28367
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iancha1992 merged 1 commit intobazelbuild:release-8.6.0from Jan 23, 2026
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[8.6.0] Compensate for Windows filesystems lacking junction support#28367iancha1992 merged 1 commit intobazelbuild:release-8.6.0from
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**Problem**: Bazel fails completely on Windows when using filesystems that don't support junction/reparse point operations (e.g., virtiofs, VirtualBox shared folders, network drives, RAM disks). The fatal error occurs when `ReadSymlinkOrJunction` fails during path resolution (e.g., when Starlark code calls `.realpath`): "Cannot read link: DeviceIoControl: Incorrect function". This causes build analysis to abort completely. Additionally, `CreateJunction` failures when creating convenience symlinks produce cryptic error messages, though these were already non-fatal warnings. Both fail because `DeviceIoControl` returns `ERROR_INVALID_FUNCTION` when the filesystem doesn't implement `FSCTL_GET_REPARSE_POINT` or `FSCTL_SET_REPARSE_POINT` operations. **Proposed solution:** Handle `ERROR_INVALID_FUNCTION` gracefully by treating it as a "not supported" condition rather than a fatal error: 1. in `ReadSymlinkOrJunction` (`file.cc`:592): return `kNotALink` instead of `kError` when `ERROR_INVALID_FUNCTION` occurs. This allows path resolution to continue for non-symlink paths on unsupported filesystems. 2. in `CreateJunction` (`file.cc`:461): return new `kNotSupported` result code when `ERROR_INVALID_FUNCTION` occurs. This produces clear "filesystem does not support junctions" warnings instead of cryptic "Incorrect function" messages. This improves UX but doesn't change behavior (these failures were already non-fatal). This follows the try-first, fallback-on-error pattern (EAFP) used by other major projects when handling unsupported filesystem operations. **Prior art:** - Rust (rust-lang/rust#138133): checks `ERROR_INVALID_FUNCTION`, `ERROR_NOT_SUPPORTED`, and `ERROR_INVALID_PARAMETER` for filesystem operation fallbacks in `std::fs::rename`. - Microsoft STL (microsoft/STL#2077): handles junctions and reparse point errors including `ERROR_INVALID_PARAMETER` with robust fallback logic in `filesystem.cpp`. - Go (golang/go#20506): uses fallback strategies when symlink APIs are unavailable on different Windows versions. - WinFsp (winfsp/winfsp#88): documents that `ERROR_INVALID_FUNCTION` indicates `STATUS_NOT_IMPLEMENTED` for unsupported operations. - Microsoft Learn: recommends checking `FILE_SUPPORTS_REPARSE_POINTS` flag via `GetVolumeInformation`, but try-catch approach is simpler and handles edge cases where detection succeeds but operations fail. **Impact**: - enables Bazel to work on virtiofs, VirtualBox shared folders, RAM disks, and other filesystems that don't support Windows junction operations. - convenience symlinks (bazel-bin, bazel-out, etc.) still won't be created, but now with clearer error messages. **Limitations**: Full junction support would require filesystem-level changes (e.g., virtiofs driver improvements). **Testing:** Tested on Windows 11 VM with host directory mounted via virtiofs, with [rules_pkg](https://github.com/bazelbuild/rules_pkg/blob/6cdaba69ee76463b2b8e97e8d243dbb6115c3aee/toolchains/git/git_configure.bzl#L40). Before change: build analysis aborted with "Cannot read link" fatal error. After change: builds complete successfully with clearer warnings about unsupported junctions for convenience symlinks. Closes bazelbuild#27598. PiperOrigin-RevId: 833360316 Change-Id: I3751602b2bd793c1cee75b7b66fa73c955a72517
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This pull request addresses a fatal error in Bazel on Windows when using filesystems that lack junction support, such as virtiofs. The changes gracefully handle ERROR_INVALID_FUNCTION from DeviceIoControl calls. In ReadSymlinkOrJunction, this error is now treated as if the path is not a link, preventing build analysis from aborting. In CreateJunction, it provides a clearer warning message. The changes are well-contained, logical, and consistent across the native C++ and Java layers. The approach is sound and aligns with practices in other major projects. I find no issues with the implementation.
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…28367) **Problem**: Bazel fails completely on Windows when using filesystems that don't support junction/reparse point operations (e.g., virtiofs, VirtualBox shared folders, network drives, RAM disks). The fatal error occurs when `ReadSymlinkOrJunction` fails during path resolution (e.g., when Starlark code calls `.realpath`): "Cannot read link: DeviceIoControl: Incorrect function". This causes build analysis to abort completely. Additionally, `CreateJunction` failures when creating convenience symlinks produce cryptic error messages, though these were already non-fatal warnings. Both fail because `DeviceIoControl` returns `ERROR_INVALID_FUNCTION` when the filesystem doesn't implement `FSCTL_GET_REPARSE_POINT` or `FSCTL_SET_REPARSE_POINT` operations. **Proposed solution:** Handle `ERROR_INVALID_FUNCTION` gracefully by treating it as a "not supported" condition rather than a fatal error: 1. in `ReadSymlinkOrJunction` (`file.cc`:592): return `kNotALink` instead of `kError` when `ERROR_INVALID_FUNCTION` occurs. This allows path resolution to continue for non-symlink paths on unsupported filesystems. 2. in `CreateJunction` (`file.cc`:461): return new `kNotSupported` result code when `ERROR_INVALID_FUNCTION` occurs. This produces clear "filesystem does not support junctions" warnings instead of cryptic "Incorrect function" messages. This improves UX but doesn't change behavior (these failures were already non-fatal). This follows the try-first, fallback-on-error pattern (EAFP) used by other major projects when handling unsupported filesystem operations. **Prior art:** - Rust (rust-lang/rust#138133): checks `ERROR_INVALID_FUNCTION`, `ERROR_NOT_SUPPORTED`, and `ERROR_INVALID_PARAMETER` for filesystem operation fallbacks in `std::fs::rename`. - Microsoft STL (microsoft/STL#2077): handles junctions and reparse point errors including `ERROR_INVALID_PARAMETER` with robust fallback logic in `filesystem.cpp`. - Go (golang/go#20506): uses fallback strategies when symlink APIs are unavailable on different Windows versions. - WinFsp (winfsp/winfsp#88): documents that `ERROR_INVALID_FUNCTION` indicates `STATUS_NOT_IMPLEMENTED` for unsupported operations. - Microsoft Learn: recommends checking `FILE_SUPPORTS_REPARSE_POINTS` flag via `GetVolumeInformation`, but try-catch approach is simpler and handles edge cases where detection succeeds but operations fail. **Impact**: - enables Bazel to work on virtiofs, VirtualBox shared folders, RAM disks, and other filesystems that don't support Windows junction operations. - convenience symlinks (bazel-bin, bazel-out, etc.) still won't be created, but now with clearer error messages. **Limitations**: Full junction support would require filesystem-level changes (e.g., virtiofs driver improvements). **Testing:** Tested on Windows 11 VM with host directory mounted via virtiofs, with [rules_pkg](https://github.com/bazelbuild/rules_pkg/blob/6cdaba69ee76463b2b8e97e8d243dbb6115c3aee/toolchains/git/git_configure.bzl#L40). Before change: build analysis aborted with "Cannot read link" fatal error. After change: builds complete successfully with clearer warnings about unsupported junctions for convenience symlinks. Closes #27598. PiperOrigin-RevId: 833360316 Change-Id: I3751602b2bd793c1cee75b7b66fa73c955a72517 Commit dab96fc Co-authored-by: Rgis Desgroppes <[email protected]>
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### What does this PR do? Bump `.bazelversion` from 8.5.1 to 8.6.0. ### Motivation Selected changes between 8.5.1 and 8.6.0: - Fix visibility for implicit deps of parent rules (bazelbuild/bazel#28722) - Force rctx.{download_and,}extract to create user-readable files (bazelbuild/bazel#28551) - Fix disk cache failures on concurrent read-write access on Windows (bazelbuild/bazel#28529) - Add a target_type argument to ctx.actions.symlink (bazelbuild/bazel#28538) - Compensate for Windows filesystems lacking junction support (bazelbuild/bazel#28367) (our fix) - Add short_uncached and detailed_uncached options to --test_summary (bazelbuild/bazel#28343) - Add --experimental_strict_repo_env option (bazelbuild/bazel#28189) - Make overlaid files executable in http_archive (bazelbuild/bazel#28277) - Add bazel mod show_repo --all_repos and --all_visible_repos (bazelbuild/bazel#28012) - Enable --experimental_retain_test_configuration_across_testonly (bazelbuild/bazel#28115) - Add option to continue with local execution if remote cache is unavailable (bazelbuild/bazel#28001)
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### What does this PR do? Bump `.bazelversion` from 8.5.1 to 8.6.0. ### Motivation Selected changes between 8.5.1 and 8.6.0: - Fix visibility for implicit deps of parent rules (bazelbuild/bazel#28722) - Force rctx.{download_and,}extract to create user-readable files (bazelbuild/bazel#28551) - Fix disk cache failures on concurrent read-write access on Windows (bazelbuild/bazel#28529) - Add a target_type argument to ctx.actions.symlink (bazelbuild/bazel#28538) - Compensate for Windows filesystems lacking junction support (bazelbuild/bazel#28367) (our fix) - Add short_uncached and detailed_uncached options to --test_summary (bazelbuild/bazel#28343) - Add --experimental_strict_repo_env option (bazelbuild/bazel#28189) - Make overlaid files executable in http_archive (bazelbuild/bazel#28277) - Add bazel mod show_repo --all_repos and --all_visible_repos (bazelbuild/bazel#28012) - Enable --experimental_retain_test_configuration_across_testonly (bazelbuild/bazel#28115) - Add option to continue with local execution if remote cache is unavailable (bazelbuild/bazel#28001)
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### What does this PR do? Bump `bazel` version from 8.5.1 to 8.6.0 to benefit from a series of improvements and fixes. Ours (bazelbuild/bazel#28367) allows to re-enable "convenience symlinks" for Windows users and makes [`path.realpath`](https://bazel.build/rules/lib/builtins/path#realpath) succeed when sharing a folder between a Linux host and a Windows VM. ### Motivation Selected changes between 8.5.1 and 8.6.0: - 💡 bazelbuild/bazel#28001 - bazelbuild/bazel#28012 - 💡 bazelbuild/bazel#28189 - bazelbuild/bazel#28277 - bazelbuild/bazel#28343 - 🐕 bazelbuild/bazel#28367 - bazelbuild/bazel#28529 - bazelbuild/bazel#28538 - bazelbuild/bazel#28551 - bazelbuild/bazel#28722 Co-authored-by: regis.desgroppes <[email protected]>
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Problem:
Bazel fails completely on Windows when using filesystems that don't support junction/reparse point operations (e.g., virtiofs, VirtualBox shared folders, network drives, RAM disks).
The fatal error occurs when
ReadSymlinkOrJunctionfails during path resolution (e.g., when Starlark code calls.realpath): "Cannot read link: DeviceIoControl: Incorrect function".This causes build analysis to abort completely.
Additionally,
CreateJunctionfailures when creating convenience symlinks produce cryptic error messages, though these were already non-fatal warnings.Both fail because
DeviceIoControlreturnsERROR_INVALID_FUNCTIONwhen the filesystem doesn't implementFSCTL_GET_REPARSE_POINTorFSCTL_SET_REPARSE_POINToperations.Proposed solution:
Handle
ERROR_INVALID_FUNCTIONgracefully by treating it as a "not supported" condition rather than a fatal error:ReadSymlinkOrJunction(file.cc:592): returnkNotALinkinstead ofkErrorwhenERROR_INVALID_FUNCTIONoccurs. This allows path resolution to continue for non-symlink paths on unsupported filesystems.CreateJunction(file.cc:461): return newkNotSupportedresult code whenERROR_INVALID_FUNCTIONoccurs. This produces clear "filesystem does not support junctions" warnings instead of cryptic "Incorrect function" messages. This improves UX but doesn't change behavior (these failures were already non-fatal).This follows the try-first, fallback-on-error pattern (EAFP) used by other major projects when handling unsupported filesystem operations.
Prior art:
ERROR_INVALID_FUNCTION,ERROR_NOT_SUPPORTED, andERROR_INVALID_PARAMETERfor filesystem operation fallbacks instd::fs::rename.<filesystem>:read_symlinkdoes not work with junctions microsoft/STL#2077): handles junctions and reparse point errors includingERROR_INVALID_PARAMETERwith robust fallback logic infilesystem.cpp.ERROR_INVALID_FUNCTIONindicatesSTATUS_NOT_IMPLEMENTEDfor unsupported operations.FILE_SUPPORTS_REPARSE_POINTSflag viaGetVolumeInformation, but try-catch approach is simpler and handles edge cases where detection succeeds but operations fail.Impact:
Limitations:
Full junction support would require filesystem-level changes (e.g., virtiofs driver improvements).
Testing:
Tested on Windows 11 VM with host directory mounted via virtiofs, with rules_pkg. Before change: build analysis aborted with "Cannot read link" fatal error. After change: builds complete successfully with clearer warnings about unsupported junctions for convenience symlinks.
Closes #27598.
PiperOrigin-RevId: 833360316
Change-Id: I3751602b2bd793c1cee75b7b66fa73c955a72517
Commit dab96fc