Export of internal Abseil changes

--
a85860e450815776c8753f34348f41ab0e918d36 by Gennadiy Rozental <rogeeff@google.com>:

Rename ComamndLineFlag's interface SetFromString as ParseFrom.

This is the name approved by C++ API review.

PiperOrigin-RevId: 301543521

--
28f31bae2a136854fd89f0a32f281d12a40f702c by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 301524919

--
a68da3d6fbbca4c5f41a20e99ed72bb1c5dd1165 by Abseil Team <absl-team@google.com>:

Introduce absl::base_internal::StrError, a portability wrapper around the various thread-safe alternatives to C89's strerror.

PiperOrigin-RevId: 301513962

--
92ccac3b6eb18cb41cddedbfdab53b9ad481505d by Andy Getzendanner <durandal@google.com>:

Introduce absl::base_internal::StrError, a portability wrapper around the various thread-safe alternatives to C89's strerror.

PiperOrigin-RevId: 301493389

--
8e196def47c250941202840d6a1de686d681cd3e by Abseil Team <absl-team@google.com>:

Internal change

PiperOrigin-RevId: 301363394

--
dd1dcffa6c47675ba4d198730a301bd408142298 by Gennadiy Rozental <rogeeff@google.com>:

Add validation for size of void* to match the size of free function pointer.

PiperOrigin-RevId: 301341331
GitOrigin-RevId: a85860e450815776c8753f34348f41ab0e918d36
Change-Id: I27c9d1365d3c9b4328d1587ab3ac38e2d09a6ec2

absl/base/internal/strerror.cc

@@ -0,0 +1,75 @@
+// Copyright 2020 The Abseil Authors.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      https://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "absl/base/internal/strerror.h"
+
+#include <cerrno>
+#include <cstddef>
+#include <cstdio>
+#include <cstring>
+#include <string>
+#include <type_traits>
+
+#include "absl/base/attributes.h"
+#include "absl/base/internal/errno_saver.h"
+
+namespace absl {
+ABSL_NAMESPACE_BEGIN
+namespace base_internal {
+namespace {
+const char* StrErrorAdaptor(int errnum, char* buf, size_t buflen) {
+#if defined(_WIN32)
+  int rc = strerror_s(buf, buflen, errnum);
+  buf[buflen - 1] = '\0';  // guarantee NUL termination
+  if (rc == 0 && strncmp(buf, "Unknown error", buflen) == 0) *buf = '\0';
+  return buf;
+#else
+#if defined(__GLIBC__) || defined(__APPLE__)
+  // Use the BSD sys_errlist API provided by GNU glibc and others to
+  // avoid any need to copy the message into the local buffer first.
+  if (0 <= errnum && errnum < sys_nerr) {
+    if (const char* p = sys_errlist[errnum]) {
+      return p;
+    }
+  }
+#endif
+  // The type of `ret` is platform-specific; both of these branches must compile
+  // either way but only one will execute on any given platform:
+  auto ret = strerror_r(errnum, buf, buflen);
+  if (std::is_same<decltype(ret), int>::value) {
+    // XSI `strerror_r`; `ret` is `int`:
+    if (ret) *buf = '\0';
+    return buf;
+  } else {
+    // GNU `strerror_r`; `ret` is `char *`:
+    return reinterpret_cast<const char*>(ret);
+  }
+#endif
+}
+}  // namespace
+
+std::string StrError(int errnum) {
+  absl::base_internal::ErrnoSaver errno_saver;
+  char buf[100];
+  const char* str = StrErrorAdaptor(errnum, buf, sizeof buf);
+  if (*str == '\0') {
+    snprintf(buf, sizeof buf, "Unknown error %d", errnum);
+    str = buf;
+  }
+  return str;
+}
+
+}  // namespace base_internal
+ABSL_NAMESPACE_END
+}  // namespace absl