build: libbacktrace: fix windows msvc integration

  libbacktrace was enabled for Windows/MSVC through a custom CMake path,
  but that path diverged from upstream configure.ac assumptions and broke
  Windows builds after the b9e4006 update.

  Fix the Windows build wiring to match upstream behavior:

  - select unwind backend by compiler:
      - backtrace.c for toolchains with _Unwind_* support
      - nounwind.c for MSVC cl.exe
  - generate a local unistd.h compatibility shim in
    libbacktrace_config for MSVC (open/read/close/lseek/getpid)
  - correct generated backtrace-supported.h values on Windows:
      - set BACKTRACE_SUPPORTED from selected unwind backend
      - set BACKTRACE_SUPPORTS_THREADS=0 for MSVC config
      - set BACKTRACE_SUPPORTS_DATA=0 for PE/COFF

  This keeps libbacktrace enabled on Windows while fixing the MSVC
  build failure and aligning metadata/macros with upstream semantics.

Signed-off-by: Eduardo Silva <eduardo@chronosphere.io>

Author: edsiper

cmake/libbacktrace.cmake

@@ -28,6 +28,16 @@ if(NOT LIBBACKTRACE_FOUND)
     # autoconf/configure doesn't work with MSVC, so we build directly
     message(STATUS "libbacktrace: building directly with CMake for Windows/MSVC")
 
+    # Match configure.ac behavior for unwind support:
+    # - GCC/clang with libunwind API -> backtrace.c
+    # - MSVC cl.exe -> nounwind.c
+    set(LIBBACKTRACE_BACKTRACE_SOURCE ${LIBBACKTRACE_SOURCE_DIR}/backtrace.c)
+    set(LIBBACKTRACE_BACKTRACE_SUPPORTED 1)
+    if(CMAKE_C_COMPILER_ID STREQUAL "MSVC")
+      set(LIBBACKTRACE_BACKTRACE_SOURCE ${LIBBACKTRACE_SOURCE_DIR}/nounwind.c)
+      set(LIBBACKTRACE_BACKTRACE_SUPPORTED 0)
+    endif()
+
     # Core source files (always needed)
     set(LIBBACKTRACE_CORE_SOURCES
       ${LIBBACKTRACE_SOURCE_DIR}/atomic.c
@@ -45,7 +55,7 @@ if(NOT LIBBACKTRACE_FOUND)
     # - View: read.c (file reading, not mmap for Windows)
     # - Alloc: alloc.c (standard alloc, not mmap for Windows)
     set(LIBBACKTRACE_PLATFORM_SOURCES
-      ${LIBBACKTRACE_SOURCE_DIR}/backtrace.c
+      ${LIBBACKTRACE_BACKTRACE_SOURCE}
       ${LIBBACKTRACE_SOURCE_DIR}/pecoff.c
       ${LIBBACKTRACE_SOURCE_DIR}/read.c
       ${LIBBACKTRACE_SOURCE_DIR}/alloc.c
@@ -121,6 +131,51 @@ if(NOT LIBBACKTRACE_FOUND)
       "#endif /* CONFIG_H */\n"
     )
 
+    # Create unistd.h compatibility wrapper for MSVC.
+    # libbacktrace sources include <unistd.h> directly, but MSVC does not
+    # provide that header.
+    set(LIBBACKTRACE_UNISTD_H "${LIBBACKTRACE_CONFIG_DIR}/unistd.h")
+    file(WRITE ${LIBBACKTRACE_UNISTD_H}
+      "/* unistd.h compatibility for Windows/MSVC - generated by CMake */\n"
+      "#ifndef LIBBACKTRACE_UNISTD_H\n"
+      "#define LIBBACKTRACE_UNISTD_H\n"
+      "\n"
+      "#include <io.h>\n"
+      "#include <process.h>\n"
+      "#include <sys/types.h>\n"
+      "\n"
+      "#ifndef _SSIZE_T_DEFINED\n"
+      "#ifdef _WIN64\n"
+      "typedef __int64 ssize_t;\n"
+      "#else\n"
+      "typedef int ssize_t;\n"
+      "#endif\n"
+      "#define _SSIZE_T_DEFINED\n"
+      "#endif\n"
+      "\n"
+      "#ifndef read\n"
+      "#define read _read\n"
+      "#endif\n"
+      "\n"
+      "#ifndef close\n"
+      "#define close _close\n"
+      "#endif\n"
+      "\n"
+      "#ifndef open\n"
+      "#define open _open\n"
+      "#endif\n"
+      "\n"
+      "#ifndef lseek\n"
+      "#define lseek _lseeki64\n"
+      "#endif\n"
+      "\n"
+      "#ifndef getpid\n"
+      "#define getpid _getpid\n"
+      "#endif\n"
+      "\n"
+      "#endif /* LIBBACKTRACE_UNISTD_H */\n"
+    )
+
     # Create backtrace-supported.h for Windows (normally generated by autoconf)
     set(LIBBACKTRACE_SUPPORTED_H "${LIBBACKTRACE_CONFIG_DIR}/backtrace-supported.h")
     file(WRITE ${LIBBACKTRACE_SUPPORTED_H}
@@ -131,8 +186,8 @@ if(NOT LIBBACKTRACE_FOUND)
       "   should work, 0 if it will not.  Libraries may #include this to make\n"
       "   other arrangements.  */\n"
       "\n"
-      "/* Windows with PE/COFF format supports backtrace */\n"
-      "#define BACKTRACE_SUPPORTED 1\n"
+      "/* Stack unwind availability depends on the compiler runtime. */\n"
+      "#define BACKTRACE_SUPPORTED ${LIBBACKTRACE_BACKTRACE_SUPPORTED}\n"
       "\n"
       "/* BACKTRACE_USES_MALLOC will be #define'd as 1 if the backtrace\n"
       "   library will call malloc as it works, 0 if it will call mmap\n"
@@ -141,13 +196,13 @@ if(NOT LIBBACKTRACE_FOUND)
       "\n"
       "/* BACKTRACE_SUPPORTS_THREADS will be #define'd as 1 if the backtrace\n"
       "   library is configured with threading support, 0 if not.  */\n"
-      "/* Windows supports threads via Windows API */\n"
-      "#define BACKTRACE_SUPPORTS_THREADS 1\n"
+      "/* __sync/__atomic are not available under MSVC cl.exe. */\n"
+      "#define BACKTRACE_SUPPORTS_THREADS 0\n"
       "\n"
       "/* BACKTRACE_SUPPORTS_DATA will be #defined'd as 1 if the backtrace_syminfo\n"
       "   will work for variables.  It will always work for functions.  */\n"
-      "/* Windows PE/COFF format supports data symbols */\n"
-      "#define BACKTRACE_SUPPORTS_DATA 1\n"
+      "/* configure.ac sets this to 'no' for PE/COFF. */\n"
+      "#define BACKTRACE_SUPPORTS_DATA 0\n"
     )
 
     # Build libbacktrace as a static library