[libc] fix line buffered empty file writes

[michaelrj]

Summary

Previously, any line buffered write of size 0 would cause an error.
The variable used to track the index of the last newline started at
the size of the write - 1, which underflowed. Now it's handled properly,
and a test has been added to prevent regressions.

libc/src/__support/File/file.cpp

size_t File::write_unlocked_fbf(const void *data, size_t len) {
   }
 
   return len;
 }
 
 size_t File::write_unlocked_lbf(const void *data, size_t len) {
   constexpr char NEWLINE_CHAR = '\n';
   size_t last_newline = len;
-  for (size_t i = len - 1; i > 0; --i) {
-    if (static_cast<const char *>(data)[i] == NEWLINE_CHAR) {
-      last_newline = i;
+  for (size_t i = len; i > 1; --i) {
+    if (static_cast<const char *>(data)[i - 1] == NEWLINE_CHAR) {
+      last_newline = i - 1;
       break;
     }
   }
 
   // If there is no newline, treat this as fully buffered.
   if (last_newline == len) {
     return write_unlocked_fbf(data, len);
   }

libc/test/src/__support/File/file_test.cpp

TEST(LlvmLibcFileTest, ZeroLengthBuffer) {
   ASSERT_STREQ(f_fbf->get_str(), WRITE_DATA);
   ASSERT_STREQ(f_lbf->get_str(), WRITE_DATA);
   ASSERT_STREQ(f_nbf->get_str(), WRITE_DATA);
 
   ASSERT_EQ(f_fbf->close(), 0);
   ASSERT_EQ(f_lbf->close(), 0);
   ASSERT_EQ(f_nbf->close(), 0);
 }
+
+TEST(LlvmLibcFileTest, WriteNothing) {
+  const char WRITE_DATA[] = "";
+  constexpr size_t WRITE_SIZE = 0;
+  constexpr size_t FILE_BUFFER_SIZE = 5;
+  char file_buffer_fbf[FILE_BUFFER_SIZE];
+  char file_buffer_lbf[FILE_BUFFER_SIZE];
+  char file_buffer_nbf[FILE_BUFFER_SIZE];
+  StringFile *f_fbf =
+      new_string_file(file_buffer_fbf, FILE_BUFFER_SIZE, _IOFBF, false, "w");
+  StringFile *f_lbf =
+      new_string_file(file_buffer_lbf, FILE_BUFFER_SIZE, _IOLBF, false, "w");
+  StringFile *f_nbf =
+      new_string_file(file_buffer_nbf, FILE_BUFFER_SIZE, _IONBF, false, "w");
+
+  ASSERT_EQ(WRITE_SIZE, f_fbf->write(WRITE_DATA, WRITE_SIZE));
+  ASSERT_EQ(WRITE_SIZE, f_lbf->write(WRITE_DATA, WRITE_SIZE));
+  ASSERT_EQ(WRITE_SIZE, f_nbf->write(WRITE_DATA, WRITE_SIZE));
+
+  ASSERT_FALSE(f_fbf->error_unlocked());
+  ASSERT_FALSE(f_lbf->error_unlocked());
+  ASSERT_FALSE(f_nbf->error_unlocked());
+
+  ASSERT_EQ(f_fbf->close(), 0);
+  ASSERT_EQ(f_lbf->close(), 0);
+  ASSERT_EQ(f_nbf->close(), 0);
+}