Introduce preExecProcessAction on Darwin, rename preSpawnProcessConfigurator to preExecProcessAction on Linux #140
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iCharlesHu
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Aug 14, 2025
…gurator to preExecProcessAction on Linux This patch introduces a preExecProcessAction on Darwin, which runs immediately before posix_spawn-as-exec, just as preSpawnProcessConfigurator (now called preExecProcessAction) runs immediately before exe on Linux. The reason to introduce this API is because there is presently no way to run code _after_ fork on Darwin in the case where pre-forking occurs (which is true when setting a uid, gid, sgroups, or creating a session). This can be useful when performing operations that modify process global state and wanting to do so without affecting the parent. The reason to rename the Linux API is that these are fundamentally two different logical operations: one is modifying flags to the process spawning mechanism (CreateProcessW, posix_spawn), while the other is performing an action at a specific point in the process lifecycle (before exec). Only Darwin uses posix_spawn, so there is no opportunity to modify any flags, and preSpawnProcessConfigurator is therefore not available on Linux/Android/FreeBSD/etc. On Windows, there is no fork/exec concept, so preExecProcessAction is not available on Windows. This brings the set of APIs to: - preSpawnProcessConfigurator (Windows, Darwin) - modifies flags bitset / STARTUPINFOW, or posix_spawnattr_t / posix_spawn_file_actions_t - preExecProcessAction (Darwin, Linux/Android/FreeBSD/etc.) - runs before exec or posix_spawn-as-exec or - Windows: preSpawnProcessConfigurator - Darwin: preSpawnProcessConfigurator + preExecProcessAction - Linux/Android/FreeBSD/etc.: preExecProcessAction This means that Darwin has both preSpawnProcessConfigurator and preExecProcessAction, because the pre-forking case uses posix_spawn _and_ fork. It also give us the opportunity to introduce preSpawnProcessConfigurator on Linux and other platforms in the future if they introduce a similar POSIX_SPAWN_CLOEXEC_DEFAULT flag or POSIX otherwise standardizes a mechanism to disable file descriptor inheritance by default, since that would allow us to use posix_spawn on those platforms. Note that merely setting preExecProcessAction will also cause pre-forking to occur. Closes swiftlang#25
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This patch introduces a preExecProcessAction on Darwin, which runs immediately before posix_spawn-as-exec, just as preSpawnProcessConfigurator (now called preExecProcessAction) runs immediately before exe on Linux.
The reason to introduce this API is because there is presently no way to run code after fork on Darwin in the case where pre-forking occurs (which is true when setting a uid, gid, sgroups, or creating a session). This can be useful when performing operations that modify process global state and wanting to do so without affecting the parent.
The reason to rename the Linux API is that these are fundamentally two different logical operations: one is modifying flags to the process spawning mechanism (CreateProcessW, posix_spawn), while the other is performing an action at a specific point in the process lifecycle (before exec). Only Darwin uses posix_spawn, so there is no opportunity to modify any flags, and preSpawnProcessConfigurator is therefore not available on Linux/Android/FreeBSD/etc. On Windows, there is no fork/exec concept, so preExecProcessAction is not available on Windows.
This brings the set of APIs to:
or
This means that Darwin has both preSpawnProcessConfigurator and preExecProcessAction, because the pre-forking case uses posix_spawn and fork. It also give us the opportunity to introduce preSpawnProcessConfigurator on Linux and other platforms in the future if they introduce a similar POSIX_SPAWN_CLOEXEC_DEFAULT flag or POSIX otherwise standardizes a mechanism to disable file descriptor inheritance by default, since that would allow us to use posix_spawn on those platforms.
Note that merely setting preExecProcessAction will also cause pre-forking to occur.
Closes #25