[darwin][process]: fix errno handling and library lifetime on darwin

[shirou]

This is a follow-up PR on #2117.

Read errno through __error() resolved before the libc call, so that the dlsym and allocation performed by symbol resolution cannot overwrite it first. Apply the same to CwdWithContext, which additionally read errno without checking for failure and could surface a stale EPERM.

Share dlopen handles process-wide instead of opening and closing them per call, completing for libSystem what #1832 did for IOKit and CoreFoundation.

Stop populating ReadBytes/WriteBytes on darwin, which #2117 filled with the disk figures. They mean all I/O including cache on other platforms, so reusing the disk value there would silently change what the field means depending on the platform.

[Copilot]

Pull request overview

This PR improves Darwin process handling by making errno retrieval reliable around libc calls, sharing purego dlopen handles for the lifetime of the process to avoid dlclose-related crashes, and aligning IOCounters semantics with other platforms by not mislabeling disk-only bytes as generic I/O bytes.

Changes:

• Fix errno handling on Darwin by resolving __error() (errno location) before the libc call and reading it only after failure, while holding runtime.LockOSThread().
• Make Darwin dynamic library handles shared process-wide (no dlclose), with a thread-safe cached symbol-resolution map.
• Stop populating ReadBytes/WriteBytes on Darwin IOCounters (leave them at 0) and update docs/tests accordingly.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
process/process.go	Updates IOCounters field docs to clarify Darwin semantics vs platforms that expose all-I/O byte counts.
process/process_darwin.go	Fixes errno read timing/thread affinity for IOCountersWithContext and CwdWithContext, and stops setting generic Read/Write bytes on Darwin.
process/process_darwin_test.go	Strengthens struct layout validation and updates IOCounters tests to reflect disk-only semantics on Darwin.
internal/common/common_darwin.go	Introduces process-wide dylib handle caching (no dlclose) and thread-safe symbol caching; adds ErrnoLocation() helper.

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[Copilot]

Pull request overview

Copilot reviewed 7 out of 7 changed files in this pull request and generated 1 comment.

Comments suppressed due to low confidence (1)

internal/common/common_darwin.go:132

• CFNumberCreate was updated to accept unsafe.Pointer to avoid passing stack addresses as uintptr into purego (which can go stale on stack growth). CFNumberGetValue (and CFNumberGetValueFunc) still take valuePtr as uintptr even though it is an output buffer written by CoreFoundation, so it can suffer the same stale-address/zeroed-results issue (e.g., callers pass pointers into local structs). Consider switching CFNumberGetValue to unsafe.Pointer and updating call sites accordingly.

func (c *CoreFoundationLib) CFNumberCreate(allocator uintptr, theType int64, valuePtr unsafe.Pointer) unsafe.Pointer {
	fn := getFunc[CFNumberCreateFunc](c.library, "CFNumberCreate")
	return fn(allocator, theType, valuePtr)
}