Export of internal Abseil changes.

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5f1ab09522226336830d9ea6ef7276d37f536ac5 by Abseil Team <absl-team@google.com>:

Clarify the documentation of ABSL_MUST_USE_RESULT.

PiperOrigin-RevId: 221663609

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af4c8359a20d56369fd1dce318220cf3be03ca66 by Greg Falcon <gfalcon@google.com>:

Internal change

PiperOrigin-RevId: 221538448

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487cd09bd1942bf607080deeae38fee6ce66f294 by Eric Fiselier <ericwf@google.com>:

Work around emscripten bugs and missing features in absl/time:time_test.

The emscripten toolchain has a couple of issues that cause time_test
to fail. Specifically:

1) emscripten doesn't support signals.
2) The javascript implementation of strftime/strptime use different expansions
of '%c' that mean it doesn't round-trip.

PiperOrigin-RevId: 221523701

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5823652e6a200b97b07334bc47128dfac40e20fc by Xiaoyi Zhang <zhangxy@google.com>:

Fix MSVC compiler warning by explicitly casting to char.
Currently our MSVC build breaks with the following error:
raw_hash_set.h(406): warning C4309: 'argument': truncation of constant value

PiperOrigin-RevId: 221492585

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c5806358320711a5efbe5c523df13e14ab53a17d by Greg Falcon <gfalcon@google.com>:

Replace calls to getpagesize() with the more portable sysconf(_SC_PAGESIZE); the latter is in POSIX 1.0 and is called out in the Linux `getpagesize` man page as a more portable spelling.

PiperOrigin-RevId: 221492471

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19ffe82851072229bb7ce73f754ffe4c18e8c575 by Abseil Team <absl-team@google.com>:

Fix -Wundef error in absl/hash/internal/hash.h.

PiperOrigin-RevId: 221444120

--
b30f3d0a848563b6e4ec33f3dc085831dfabb748 by Jon Cohen <cohenjon@google.com>:

Import of CCTZ from GitHub.

PiperOrigin-RevId: 221339736
GitOrigin-RevId: 5f1ab09522226336830d9ea6ef7276d37f536ac5
Change-Id: I96223d522d98bf6616dea88eb047c2d536eeddd0

absl/base/attributes.h

@@ -402,17 +402,28 @@
 
 // ABSL_MUST_USE_RESULT
 //
-// Tells the compiler to warn about unused return values for functions declared
-// with this macro. The macro must appear as the very first part of a function
-// declaration or definition:
+// Tells the compiler to warn about unused results.
 //
-// Example:
+// When annotating a function, it must appear as the first part of the
+// declaration or definition. The compiler will warn if the return value from
+// such a function is unused:
 //
 //   ABSL_MUST_USE_RESULT Sprocket* AllocateSprocket();
+//   AllocateSprocket();  // Triggers a warning.
 //
-// This placement has the broadest compatibility with GCC, Clang, and MSVC, with
-// both defs and decls, and with GCC-style attributes, MSVC declspec, C++11
-// and C++17 attributes.
+// When annotating a class, it is equivalent to annotating every function which
+// returns an instance.
+//
+//   class ABSL_MUST_USE_RESULT Sprocket {};
+//   Sprocket();  // Triggers a warning.
+//
+//   Sprocket MakeSprocket();
+//   MakeSprocket();  // Triggers a warning.
+//
+// Note that references and pointers are not instances:
+//
+//   Sprocket* SprocketPointer();
+//   SprocketPointer();  // Does *not* trigger a warning.
 //
 // ABSL_MUST_USE_RESULT allows using cast-to-void to suppress the unused result
 // warning. For that, warn_unused_result is used only for clang but not for gcc.

absl/base/config.h

@@ -274,7 +274,8 @@
 #error ABSL_HAVE_MMAP cannot be directly set
 #elif defined(__linux__) || defined(__APPLE__) || defined(__FreeBSD__) ||   \
     defined(__ros__) || defined(__native_client__) || defined(__asmjs__) || \
-    defined(__wasm__) || defined(__Fuchsia__) || defined(__sun)
+    defined(__wasm__) || defined(__Fuchsia__) || defined(__sun) || \
+    defined(__ASYLO__)
 #define ABSL_HAVE_MMAP 1
 #endif
 
@@ -328,6 +329,8 @@
 #define ABSL_HAVE_ALARM 1
 #elif defined(_MSC_VER)
 // feature tests for Microsoft's library
+#elif defined(__EMSCRIPTEN__)
+// emscripten doesn't support signals
 #elif defined(__native_client__)
 #else
 // other standard libraries

absl/base/internal/direct_mmap.h

@@ -75,7 +75,7 @@ inline void* DirectMmap(void* start, size_t length, int prot, int flags, int fd,
   // On these architectures, implement mmap with mmap2.
   static int pagesize = 0;
   if (pagesize == 0) {
-    pagesize = getpagesize();
+    pagesize = sysconf(_SC_PAGESIZE);
   }
   if (offset < 0 || offset % pagesize != 0) {
     errno = EINVAL;

absl/base/internal/low_level_alloc.cc

@@ -208,7 +208,7 @@ struct LowLevelAlloc::Arena {
   int32_t allocation_count GUARDED_BY(mu);
   // flags passed to NewArena
   const uint32_t flags;
-  // Result of getpagesize()
+  // Result of sysconf(_SC_PAGESIZE)
   const size_t pagesize;
   // Lowest power of two >= max(16, sizeof(AllocList))
   const size_t roundup;
@@ -325,7 +325,7 @@ size_t GetPageSize() {
   GetSystemInfo(&system_info);
   return std::max(system_info.dwPageSize, system_info.dwAllocationGranularity);
 #else
-  return getpagesize();
+  return sysconf(_SC_PAGESIZE);
 #endif
 }