Speed up Guid.NewGuid() on Linux

[reedz]

Fixes #13628.

This PR implements two optimizations:

• getrandom() syscall (if available) instead of reading from /dev/urandom is ~14% quicker >
• fetching & caching GUIDs in batches in thread local storage for future use via single getrandom() call as performed in Random.Shared

Benchmark dot net benchmark is showing the following changes:

Runtime	Mean	Improvement
Linux (before)	614.2 ns	baseline
Linux (after)	61.16 ns	~10x faster

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[Copilot]

Pull request overview

This PR optimizes Guid.NewGuid() on Unix, primarily targeting Linux, by reducing syscall overhead and reusing entropy to significantly improve performance while maintaining cryptographic-strength randomness.

Changes:

• Teach the native random infrastructure to use getrandom() (when available) instead of /dev/urandom, reducing per-call overhead for cryptographically secure random bytes.
• Add CMake configuration and header wiring to detect and expose the availability of getrandom.
• Reimplement Guid.NewGuid() on Unix to use a per-thread cache of GUIDs filled in batches via a single secure-random call, including logic to correctly set RFC 4122 version and variant bits for each cached GUID.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 1 comment.

File	Description
src/native/minipal/random.c	Adds a HAVE_GETRANDOM-guarded implementation of minipal_get_cryptographically_secure_random_bytes using the getrandom() syscall, looping until the requested buffer is fully populated and handling EINTR, to provide a faster secure RNG backend than /dev/urandom.
src/native/minipal/minipalconfig.h.in	Introduces the HAVE_GETRANDOM configuration macro so native code can be conditionally compiled against getrandom() support.
src/native/minipal/configure.cmake	Extends the CMake configuration to probe for getrandom in sys/random.h, defining HAVE_GETRANDOM when present so the new native path in random.c is enabled on supported Unix platforms.
src/libraries/System.Private.CoreLib/src/System/Guid.Unix.cs	Replaces the per-call secure RNG usage with a [ThreadStatic] cache of 64 GUIDs, filled via a single Interop.GetCryptographicallySecureRandomBytes call and post-processed to set version/variant bits, and updates NewGuid() to draw from this cache (with a WASI-specific non-cached fallback).

[Copilot]

System.Runtime.InteropServices does not appear to be used in this file anymore; consider removing this using to avoid an unnecessary import and keep the file consistent with the usual style of only including needed namespaces.

Suggested change

using System.Runtime.InteropServices;

[EgorBo]

Managed side cache of secure RNG doesn't sound secure. If NewGuid becomes a bottleneck there are other strategies to make it fast

[jkotas]

1kB per thread cache just for GUIDs feels very expensive.

[jkotas]

This part of the change looks good.

[jkotas]

Would you mind reverting the contentious cache and focus this PR on just using getrandom when it is available?

[vcsjones]

Should we fall back to /dev/urandom for some errors? In particular, I was thinking should consider handling ENOSYS and EPERM. Some container environments or seccomp do, or did, not pass through that syscall.

[reedz]

Sure thing!
I've reverted managed changes, and updated the getrandom branch to fall back to /dev/urandom on ENOSYS and EPERM returns.

[stephentoub]

Related:
#111287
@vcsjones

[stephentoub]

nit: could we use the same terminology as is there below for devurandom, e.g. sMissingGetrandom?