Add Cygwin support: 10 patches for full platform port

Cygwin (POSIX on Windows/PE-COFF) was blocked by missing __CYGWIN__
guards in platform-detection lists. Each omission caused a different
failure mode: compile errors, silent data loss, ABI mismatches, or
wrong runtime behavior.

The patches fall into three categories: platform detection and feature
guards, type width corrections where pthread_t is pointer-sized instead
of int-sized, and two test accommodations for Cygwin-specific behavior.

5 patches derive from abseil/abseil-cpp PR abseil#2012 (Carlo Bramini).
The remaining 5 address issues not covered by that PR.

Build requirement: -DGTEST_HAS_PTHREAD=1 — GoogleTest's auto-detection
omits Cygwin from its pthread platform list, causing a Mutex ABI
mismatch between libgmock and test binaries.

Author: philip-h-dye

absl/base/attributes.h

@@ -143,7 +143,7 @@
     (!defined(_WIN32) ||                                          \
      (defined(__clang__) && __clang_major__ >= 9 &&               \
       !defined(ABSL_BUILD_DLL) && !defined(ABSL_CONSUME_DLL))) && \
-    !defined(__MINGW32__)
+    !defined(__MINGW32__) && !defined(__CYGWIN__)
 #undef ABSL_ATTRIBUTE_WEAK
 #define ABSL_ATTRIBUTE_WEAK __attribute__((weak))
 #define ABSL_HAVE_ATTRIBUTE_WEAK 1

absl/base/config.h

@@ -377,7 +377,8 @@ static_assert(ABSL_INTERNAL_INLINE_NAMESPACE_STR[0] != 'h' ||
     defined(__asmjs__) || defined(__EMSCRIPTEN__) || defined(__Fuchsia__) || \
     defined(__sun) || defined(__myriad2__) || defined(__HAIKU__) ||          \
     defined(__OpenBSD__) || defined(__NetBSD__) || defined(__QNX__) ||       \
-    defined(__VXWORKS__) || defined(__hexagon__) || defined(__XTENSA__)
+    defined(__VXWORKS__) || defined(__hexagon__) || defined(__XTENSA__) || \
+    defined(__CYGWIN__)
 #define ABSL_HAVE_MMAP 1
 #endif
 

absl/base/internal/low_level_alloc.h

@@ -36,7 +36,7 @@
 // LowLevelAlloc.
 #ifdef ABSL_LOW_LEVEL_ALLOC_MISSING
 #error ABSL_LOW_LEVEL_ALLOC_MISSING cannot be directly set
-#elif !defined(ABSL_HAVE_MMAP) && !defined(_WIN32)
+#elif !defined(ABSL_HAVE_MMAP) && !defined(_WIN32) && !defined(__CYGWIN__)
 #define ABSL_LOW_LEVEL_ALLOC_MISSING 1
 #endif
 
@@ -47,7 +47,7 @@
 #ifdef ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING
 #error ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING cannot be directly set
 #elif defined(_WIN32) || defined(__asmjs__) || defined(__wasm__) || \
-    defined(__hexagon__)
+    defined(__hexagon__) || defined(__CYGWIN__)
 #define ABSL_LOW_LEVEL_ALLOC_ASYNC_SIGNAL_SAFE_MISSING 1
 #endif
 

absl/base/internal/raw_logging.cc

@@ -44,7 +44,8 @@
 #if defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) || \
     defined(__hexagon__) || defined(__Fuchsia__) ||                     \
     defined(__native_client__) || defined(__OpenBSD__) ||               \
-    defined(__EMSCRIPTEN__) || defined(__ASYLO__)
+    defined(__EMSCRIPTEN__) || defined(__ASYLO__) ||                    \
+    defined(__CYGWIN__)
 
 #include <unistd.h>
 

absl/base/internal/sysinfo.cc

@@ -481,7 +481,7 @@ pid_t GetTID() {
 pid_t GetTID() {
   // `pthread_t` need not be arithmetic per POSIX; platforms where it isn't
   // should be handled above.
-  return static_cast<pid_t>(pthread_self());
+  return reinterpret_cast<pid_t>(pthread_self());
 }
 
 #endif

absl/base/internal/sysinfo.h

@@ -57,6 +57,9 @@ int NumCPUs();
 // return types of GetProcessId() and GetThreadId() are both DWORD, an unsigned
 // 32-bit type.
 using pid_t = uint32_t;
+#elif defined(__CYGWIN__)
+// On CYGWIN, this works on both i686 (deprecated) and x64 targets.
+using pid_t = uintptr_t;
 #endif
 pid_t GetTID();
 

absl/base/policy_checks.h

@@ -38,10 +38,6 @@
 // Operating System Check
 // -----------------------------------------------------------------------------
 
-#if defined(__CYGWIN__)
-#error "Cygwin is not supported."
-#endif
-
 // -----------------------------------------------------------------------------
 // Toolchain Check
 // -----------------------------------------------------------------------------

absl/log/internal/config.h

@@ -34,6 +34,11 @@ namespace log_internal {
 
 #ifdef _WIN32
 using Tid = uint32_t;
+#elif defined(__CYGWIN__)
+// Cygwin's GetTID() returns reinterpret_cast<uintptr_t>(pthread_self()),
+// which is an 8-byte pointer on x86_64. System pid_t is int (4 bytes)
+// and would truncate the thread ID, causing collisions in log output.
+using Tid = uint64_t;
 #else
 using Tid = pid_t;
 #endif

absl/log/stripping_test.cc

@@ -170,7 +170,7 @@ class StrippingTest : public ::testing::Test {
   // Opens this program's executable file.  Returns `nullptr` and writes to
   // `stderr` on failure.
   std::unique_ptr<FILE, std::function<void(FILE*)>> OpenTestExecutable() {
-#if defined(__linux__)
+#if defined(__linux__) || defined(__CYGWIN__)
     std::unique_ptr<FILE, std::function<void(FILE*)>> fp(
         fopen("/proc/self/exe", "rb"), [](FILE* fp) { fclose(fp); });
     if (!fp) {

absl/strings/charconv_test.cc

@@ -31,6 +31,13 @@
 #ifdef _MSC_FULL_VER
 #define ABSL_COMPILER_DOES_EXACT_ROUNDING 0
 #define ABSL_STRTOD_HANDLES_NAN_CORRECTLY 0
+#elif defined(__CYGWIN__)
+#define ABSL_COMPILER_DOES_EXACT_ROUNDING 1
+// Cygwin's strtod() handles NaN payloads differently for large or
+// non-numeric n-char-sequences (e.g., "abc123", very long digit strings).
+// The NaN payload encoding is unspecified per IEEE 754, so this is a
+// platform difference, not a bug.
+#define ABSL_STRTOD_HANDLES_NAN_CORRECTLY 0
 #else
 #define ABSL_COMPILER_DOES_EXACT_ROUNDING 1
 #define ABSL_STRTOD_HANDLES_NAN_CORRECTLY 1