Fix a bug in StrFormat. This issue would have been caught by any compile-time

checking but can happen for incorrect formats parsed via ParsedFormat::New.
Specifically, if a user were to add length modifiers with 'v', for example the
incorrect format string "%hv", the ParsedFormat would incorrectly be allowed.

PiperOrigin-RevId: 480183817
Change-Id: I8510c13189fdf807cdaa7f2e1b7ed9fba2aaefb9

absl/strings/internal/str_format/parser.cc

@@ -202,9 +202,7 @@ const char *ConsumeConversion(const char *pos, const char *const end,
 
   auto tag = GetTagForChar(c);
 
-  if (*(pos - 1) == 'v' && *(pos - 2) != '%') {
-    return nullptr;
-  }
+  if (ABSL_PREDICT_FALSE(c == 'v' && (pos - original_pos) != 1)) return nullptr;
 
   if (ABSL_PREDICT_FALSE(!tag.is_conv())) {
     if (ABSL_PREDICT_FALSE(!tag.is_length())) return nullptr;
@@ -223,6 +221,8 @@ const char *ConsumeConversion(const char *pos, const char *const end,
       conv->length_mod = length_mod;
     }
     tag = GetTagForChar(c);
+
+    if (ABSL_PREDICT_FALSE(c == 'v')) return nullptr;
     if (ABSL_PREDICT_FALSE(!tag.is_conv())) return nullptr;
   }
 

absl/strings/internal/str_format/parser_test.cc

@@ -110,10 +110,13 @@ TEST_F(ConsumeUnboundConversionTest, ConsumeSpecification) {
     {__LINE__, "ba",   "",     "ba"},  // 'b' is invalid
     {__LINE__, "l",    "",     "l" },  // just length mod isn't okay
     {__LINE__, "d",    "d",    ""  },  // basic
+    {__LINE__, "v",    "v",    ""  },  // basic
     {__LINE__, "d ",   "d",    " " },  // leave suffix
     {__LINE__, "dd",   "d",    "d" },  // don't be greedy
     {__LINE__, "d9",   "d",    "9" },  // leave non-space suffix
     {__LINE__, "dzz",  "d",    "zz"},  // length mod as suffix
+    {__LINE__, "3v",   "",     "3v"},  // 'v' cannot have modifiers
+    {__LINE__, "hv",   "",     "hv"},  // 'v' cannot have modifiers
     {__LINE__, "1$*2$d", "1$*2$d", ""  },  // arg indexing and * allowed.
     {__LINE__, "0-14.3hhd", "0-14.3hhd", ""},  // precision, width
     {__LINE__, " 0-+#14.3hhd", " 0-+#14.3hhd", ""},  // flags