printf: Add support for glibc's %m format

glibc's printf family of functions support '%m' to emit the error string
that corresponds to the current value of errno. It has no corresponding
argument.

In order to take advantage of safe_strerror, PrintfFormatter::format calls
format_system_error, which has been modified to not emit the separator if
the message prefix is empty.

'm' needs to be added to the list of special characters in format-test.cc's
check_unknown_types since it would otherwise always be valid in the format
string.

The AppVeyor mingw test shows that errno is being reset to zero during the
processing of '%m', so it needs to be explicitly preserved. This doesn't
seem to happen on Linux, MacOS, or Windows with MSVC but the preservation
code does no harm.

doc/api.rst

@@ -162,7 +162,9 @@ The following functions use `printf format string syntax
 <http://pubs.opengroup.org/onlinepubs/009695399/functions/fprintf.html>`_ with
 the POSIX extension for positional arguments. Unlike their standard
 counterparts, the ``fmt`` functions are type-safe and throw an exception if an
-argument type doesn't match its format specification.
+argument type doesn't match its format specification. They also support the
+glibc '%m' extension to output the string corresponding to the error code
+in errno.
 
 .. doxygenfunction:: printf(CStringRef, ArgList)
 

fmt/format.cc

@@ -385,7 +385,10 @@ FMT_FUNC void format_system_error(
       char *system_message = &buffer[0];
       int result = safe_strerror(error_code, system_message, buffer.size());
       if (result == 0) {
-        out << message << ": " << system_message;
+	  if (message.size())
+	    out << message << ": " << system_message;
+	  else
+	    out << system_message;
         return;
       }
       if (result != ERANGE)

fmt/printf.h

@@ -12,6 +12,7 @@
 
 #include <algorithm>  // std::fill_n
 #include <limits>     // std::numeric_limits
+#include <cerrno>     // errno
 
 #include "ostream.h"
 
@@ -197,6 +198,18 @@ class WidthHandler : public ArgVisitor<WidthHandler, unsigned> {
     return static_cast<unsigned>(width);
   }
 };
+
+class ErrnoPreserver {
+  const int saved_errno_;
+public:
+  ErrnoPreserver() : saved_errno_(errno) {}
+  ~ErrnoPreserver() {
+    errno = saved_errno_;
+  }
+  int get() const {
+    return saved_errno_;
+  }
+};
 }  // namespace internal
 
 /**
@@ -431,6 +444,21 @@ void PrintfFormatter<Char, AF>::format(BasicCStringRef<Char> format_str) {
     // Parse argument index, flags and width.
     unsigned arg_index = parse_header(s, spec);
 
+    if (*s == 'm') {
+      // Something in here is changing errno in the mingw build. We
+      // ought to preserve it anyway, even if an exception is thrown.
+      internal::ErrnoPreserver errno_preserver;
+
+      // This dance is necessary because this function is templated on
+      // the character type but format_system_error is not.
+      MemoryWriter char_writer;
+      format_system_error(char_writer, errno_preserver.get(), "");
+      AF(writer_, spec).visit_cstring(char_writer.c_str());
+
+      start = ++s;
+      continue;
+    }
+
     // Parse precision.
     if (*s == '.') {
       ++s;

test/format-test.cc

@@ -1105,7 +1105,7 @@ template <typename T>
 void check_unknown_types(
     const T &value, const char *types, const char *type_name) {
   char format_str[BUFFER_SIZE], message[BUFFER_SIZE];
-  const char *special = ".0123456789}";
+  const char *special = ".0123456789}m";
   for (int i = CHAR_MIN; i <= CHAR_MAX; ++i) {
     char c = static_cast<char>(i);
     if (std::strchr(types, c) || std::strchr(special, c) || !c) continue;

test/printf-test.cc

@@ -51,6 +51,10 @@ std::string make_positional(fmt::StringRef format) {
     << "format: " << format; \
   EXPECT_EQ(expected_output, fmt::sprintf(make_positional(format), arg))
 
+#define EXPECT_PRINTF_NO_ARG(expected_output, format) \
+  EXPECT_EQ(expected_output, fmt::sprintf(format)) \
+    << "format: " << format;
+
 TEST(PrintfTest, NoArgs) {
   EXPECT_EQ("test", fmt::sprintf("test"));
 }
@@ -472,6 +476,18 @@ TEST(PrintfTest, Enum) {
   EXPECT_PRINTF("42", "%d", A);
 }
 
+TEST(PrintfTest, ErrorMessage) {
+  errno = ENOENT;
+
+  EXPECT_PRINTF_NO_ARG("No such file or directory", "%m");
+  EXPECT_PRINTF_NO_ARG("No such file or directory     ", "%-30m");
+  EXPECT_PRINTF_NO_ARG("     No such file or directory", "%30m");
+
+  EXPECT_PRINTF_NO_ARG(L"No such file or directory", L"%m");
+  EXPECT_PRINTF_NO_ARG(L"No such file or directory     ", L"%-30m");
+  EXPECT_PRINTF_NO_ARG(L"     No such file or directory", L"%30m");
+}
+
 #if FMT_USE_FILE_DESCRIPTORS
 TEST(PrintfTest, Examples) {
   const char *weekday = "Thursday";