The bind mounting scheme used with the Linux sandbox' hermetic `/tmp` feature is modified to preserve all paths as they are outside the sandbox, which removes the need to rewrite paths when staging inputs into and, crucially, moving outputs out of the sandbox. Source roots and output base paths under `/tmp` are now treated just like any user-specified bind mount under `/tmp`: They are mounted under the hermetic tmp directory with their path relativized against `/tmp` before the hermetic tmp directory is mounted as `/tmp` as the final step. There is one caveat compared to user-specified mounts: Source roots, which may themselves not lie under `/tmp`, can be symlinks to directories under `/tmp` (e.g., when they arise from a `local_repository`). To handle this situation in the common case, all parent directories of package path entries (up to direct children of `/tmp`) are mounted into the sandbox. If users use `local_repository`s with fixed target paths under `/tmp`, they will need to specify `--sandbox_add_mount_pair`. Overlayfs has been considered as an alternative to this approach, but ultimately doesn't seem to work for this use case since its `lowerpath`, which would be `/tmp`, is not allowed to have child mounts from a different user namespace (see https://unix.stackexchange.com/questions/776030/mounting-overlayfs-in-a-user-namespace-with-child-mounts). However, this is exactly the situation created by a Bazel-in-Bazel test and can also arise if the user has existing mounts under `/tmp` when using Bazel (e.g. the JetBrains toolbox on Linux uses such mounts). This replaces and mostly reverts the following commits, but keeps their tests: * bf6ebe9 * fb6658c * bc1d9d3 * 1829883 * 70691f2 * a556969 * 8e32f44 (had its test lost in an incorrect merge conflict resolution, this PR adds it back) Fixes #20533 Work towards #20753 Fixes #21215 Fixes #22117 Fixes #22226 Fixes #22290 RELNOTES: Paths in the Linux sandbox are now again identical to those outside the sandbox, even with `--incompatible_sandbox_hermetic_tmp`. Closes #22001. PiperOrigin-RevId: 634381503 Change-Id: I9f7f3948c705be120c55c9b0c51204e5bea45f61 Fixes #22291

[7.2.0] Preserve paths under hermetic /tmp in the sandbox #22291

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[7.2.0] Preserve paths under hermetic `/tmp` in the sandbox #22291