[minipal] Add getentropy() fallback for platforms without getrandom()

[lewing]

Adds getentropy() as a fallback in minipal/random.c for platforms that provide it but lack getrandom() — notably WASI (via wasi-libc → __wasi_random_get()).

Changes

• random.c — Add #elif HAVE_GETENTROPY fallback between HAVE_BCRYPT_H and HAVE_GETRANDOM in the RNG chain
• configure.cmake — Add check_symbol_exists(getentropy "unistd.h" HAVE_GETENTROPY)
• minipalconfig.h.in — Add #cmakedefine01 HAVE_GETENTROPY

Motivation

NativeAOT-LLVM targeting wasi-wasm currently fails at RandomNumberGenerator.Fill() with PlatformNotSupportedException because minipal_get_cryptographically_random_bytes returns -1 — getrandom() returns ENOSYS and /dev/urandom doesn't exist on WASI.

However, wasi-libc already provides getentropy() backed by __wasi_random_get(). The C runtime startup already uses this path (arc4random_buf → getentropy → __wasi_random_get) for stack canaries, so the symbol is linked and functional.

Uses CMake feature detection (check_symbol_exists) rather than a hardcoded __wasi__ check, so this also covers other platforms with getentropy() but not getrandom().

Fixes #126480

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[Copilot]

Pull request overview

Adds a WASI-specific entropy source to minipal so managed cryptography can obtain secure random bytes on WASI targets.

Changes:

• Add a __wasi__ branch in minipal_get_cryptographically_secure_random_bytes.
• Use getentropy() (chunked to 256 bytes per call) as the secure RNG source on WASI.

[lewing]

Updated per @am11's suggestion — now uses CMake HAVE_GETENTROPY feature detection instead of __wasi__. Added check_symbol_exists(getentropy "unistd.h" HAVE_GETENTROPY) to configure.cmake and the corresponding #cmakedefine01 to minipalconfig.h.in. The random.c fallback chain is now: arc4random_buf → BCrypt → getentropy → getrandom → /dev/urandom.

[Copilot]

Pull request overview

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.