Bug 1851154 - Provide our own implementation of C's getline for Android r=jld

10 years ago (!) Bug 914190 already choked on the fact that bionic's
getline implementation could realloc a buffer using a function call
we cannot intercept, resulting in different memory allocator being used
to allocate and free the getline buffer.

This got hit again by 1850948, causing a backout. The approach taken at
that time (use std::getline) is neither future-proof (as demonstrated by
the backout) nor always satisfying (std::string as a few limitations in
term of low-level buffer manipulation).

Provide our implementation for Android, as hinted by the original bug.

Differential Revision: https://phabricator.services.mozilla.com/D187270

mozglue/interposers/getline_interposer.cpp

@@ -0,0 +1,110 @@
+/* This Source Code Form is subject to the terms of the Mozilla Public
+ * License, v. 2.0. If a copy of the MPL was not distributed with this
+ * file, You can obtain one at http://mozilla.org/MPL/2.0/. */
+
+/*
+ * Interposing getline because of
+ * https://bugzilla.mozilla.org/show_bug.cgi?id=914190
+ */
+
+#ifdef __ANDROID__
+
+#  include <cstdlib>
+#  include <cstdio>
+#  include <cerrno>
+
+#  include <limits>
+
+namespace {
+
+// RAII on file locking.
+class FileLocker {
+  FILE* stream;
+
+ public:
+  explicit FileLocker(FILE* stream) : stream(stream) { flockfile(stream); }
+  ~FileLocker() { funlockfile(stream); }
+};
+
+ssize_t internal_getdelim(char** __restrict lineptr, size_t* __restrict n,
+                          int delim, FILE* __restrict stream) {
+  constexpr size_t n_default = 64;
+  constexpr size_t n_max =
+      std::numeric_limits<ssize_t>::max() < std::numeric_limits<size_t>::max()
+          ? (size_t)std::numeric_limits<ssize_t>::max() + 1
+          : std::numeric_limits<size_t>::max();
+  constexpr size_t n_limit = 2  * ((n_max - 1) / 3);
+
+  if (!lineptr || !n || !stream) {
+    errno = EINVAL;
+    return -1;
+  }
+
+  // Lock the file so that we can us unlocked getc in the inner loop.
+  FileLocker fl(stream);
+
+  if (!*lineptr || *n == 0) {
+    *n = n_default;
+    if (auto* new_lineptr = reinterpret_cast<char*>(realloc(*lineptr, *n))) {
+      *lineptr = new_lineptr;
+    } else {
+      errno = ENOMEM;
+      return -1;
+    }
+  }
+
+  ssize_t result;
+  size_t cur_len = 0;
+
+  while (true) {
+    // Retrieve an extra char.
+    int i = getc_unlocked(stream);
+    if (i == EOF) {
+      result = -1;
+      break;
+    }
+
+    // Eventually grow the buffer.
+    if (cur_len + 1 >= *n) {
+      size_t needed = *n >= n_limit ? n_max : 3 * *n / 2 + 1;
+
+      if (cur_len + 1 >= needed) {
+        errno = EOVERFLOW;
+        return -1;
+      }
+
+      if (auto* new_lineptr = (char*)realloc(*lineptr, needed)) {
+        *lineptr = new_lineptr;
+      } else {
+        errno = ENOMEM;
+        return -1;
+      }
+      *n = needed;
+    }
+
+    (*lineptr)[cur_len] = i;
+    cur_len++;
+
+    if (i == delim) break;
+  }
+  (*lineptr)[cur_len] = '\0';
+  return cur_len ? cur_len : result;
+}
+
+}  // namespace
+
+extern "C" {
+
+MFBT_API ssize_t getline(char** __restrict lineptr, size_t* __restrict n,
+                         FILE* __restrict stream) {
+  return internal_getdelim(lineptr, n, '\n', stream);
+}
+
+MFBT_API ssize_t getdelim(char** __restrict lineptr, size_t* __restrict n,
+                          int delim, FILE* __restrict stream) {
+  return internal_getdelim(lineptr, n, delim, stream);
+}
+
+}  // extern "C"
+
+#endif

mozglue/interposers/moz.build

@@ -9,6 +9,7 @@ DEFINES["IMPL_MFBT"] = True
 
 UNIFIED_SOURCES += [
     "env_interposer.cpp",
+    "getline_interposer.cpp",
 ]
 
 if CONFIG["MOZ_CRASHREPORTER"]: