Initial commit of the regex matcher

Wrap glibc’s regex engine to allow matching and group capture in POSIX
extended regular expressions.

It might be worth rewriting this in terms of the C++11 regex engine;
it’s more featureful and more pleasant to use, although it would require
more casting.  (C can’t represent the std::regex type, so I’d need to
use some void pointers.)

1 files changed, 192 insertions, 0 deletions

diff --git a/src/regex__FFI.cc b/src/regex__FFI.cc
new file mode 100644
index 0000000..403171f
--- /dev/null
+++ b/src/regex__FFI.cc
@@ -0,0 +1,192 @@
+// Copyright (C) 2015 the Massachusetts Institute of Technology
+//
+// 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
+//
+//     http://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 "regex__FFI.h"
+
+#include <sys/types.h>
+#include <regex.h>
+
+#include <cstring>
+
+extern "C" {
+#include <urweb/urweb_cpp.h>
+}
+
+#include "config.h"
+
+namespace {
+
+using Regex = uw_Regex__FFI_regex;
+using Match = uw_Regex__FFI_match;
+
+// Asserts a condition without crashing or releasing information about where the
+// error occurred.  This function is essential for web programming, where an
+// attacker should not be able to bring down the app by causing an assertion
+// failure.
+void Assert(uw_context* const context, const bool condition,
+            const failure_kind action, const char* const message) {
+  if (!condition) {
+    uw_error(context, action, message);
+  }
+}
+
+void Assert(uw_context* const context,
+            const bool condition, const char* const message) {
+  Assert(context, condition, FATAL, message);
+}
+
+void FinalizeRegex(void* regex, [[gnu::unused]] const int _will_retry) {
+  regfree(reinterpret_cast<regex_t*>(regex));
+}
+
+void DeleteRegex(void* regex, [[gnu::unused]] const int _will_retry) {
+  delete reinterpret_cast<regex_t*>(regex);
+}
+
+}  // namespace
+
+uw_Basis_bool uw_Regex__FFI_succeeded(
+    [[gnu::unused]] struct uw_context* _context,
+    const Match match) {
+  return match.succeeded ? uw_Basis_True : uw_Basis_False;
+}
+
+uw_Basis_int uw_Regex__FFI_n_subexpression_matches(
+    [[gnu::unused]] struct uw_context* _context,
+    const Match match) {
+  return match.n_matches;
+}
+
+uw_Basis_string uw_Regex__FFI_subexpression_match(
+    struct uw_context* context,
+    const Match match,
+    const uw_Basis_int match_index) {
+  Assert(context, match.matches[match_index].rm_so != -1,
+         "regex: match does not exist");
+  // Locate the substring in the string to match aginst.
+  const char* const substring_start =
+      match.haystack + match.matches[match_index].rm_so;
+  // Copy it into its own buffer so we can properly null-terminate it.
+  const std::size_t substring_length =
+      static_cast<std::size_t>(match.matches[match_index].rm_eo
+                               - match.matches[match_index].rm_so);
+  uw_Basis_string result = reinterpret_cast<uw_Basis_string>(
+      uw_malloc(context, substring_length + 1));
+  std::memcpy(result, substring_start, substring_length);
+  result[substring_length] = '\0';
+  return result;
+}
+
+Regex uw_Regex__FFI_compile(uw_context* const context,
+                            const uw_Basis_bool case_sensitive,
+                            const uw_Basis_string input) {
+  Regex result;
+  result.text = input;
+  // We'd like to stack-allocate the compiled field of the Regex struct--or, at
+  // least, to allocate it with uw_malloc.  Unfortunately, neither of those will
+  // work, because we need to be able to run a finalizer on it, and Ur
+  // finalizers can only reference addresses that are not managed by Ur.
+  result.compiled = new regex_t;
+  Assert(context,
+         uw_register_transactional(context, result.compiled,
+                                   nullptr, nullptr, DeleteRegex) == 0,
+         "regex: could not register DeleteRegex finalizer");
+  // Compile the regex.
+  const auto flags = REG_EXTENDED | (case_sensitive ? 0 : REG_ICASE);
+  switch (const auto regcomp_error = regcomp(result.compiled, input, flags)) {
+    case 0:
+      // Everything worked perfectly.
+      break;
+    case REG_ESPACE:
+      // We ran out of memory.
+      uw_error(context, BOUNDED_RETRY, "regex: could not allocate");
+    default:
+      // Something else happened.  Generate a nice message for the user.
+      const auto message_size =
+          regerror(regcomp_error, result.compiled, nullptr, 0);
+      char* const message =
+          reinterpret_cast<char*>(uw_malloc(context, message_size));
+      Assert(context,
+             regerror(regcomp_error, result.compiled, message,
+                      message_size) == message_size,
+             "regex: compilation failed, but error message could not be"
+             " generated");
+      uw_error(context, FATAL, "regex: compilation failed: %s", message);
+  }
+  Assert(context,
+         uw_register_transactional(context, result.compiled,
+                                   nullptr, nullptr, FinalizeRegex) == 0,
+         "regex: could not register FinalizeRegex finalizer");
+  // Give the caller the regex.
+  return result;
+}
+
+Match uw_Regex__FFI_do_match(uw_context* const context, const Regex needle,
+                             const uw_Basis_string haystack) {
+  Match result;
+  // Make a duplicate of the string to match against, so if it goes out of scope
+  // in the calling Ur code, we still have it.  TODO(bbaren): Is this necessary?
+  result.haystack =
+      reinterpret_cast<uw_Basis_string>(
+          uw_malloc(context, std::strlen(haystack)));
+  std::strcpy(result.haystack, haystack);
+  // Figure out how many groups we could have so we can allocate enough space to
+  // store the match information.
+  result.n_matches = 0;
+  for (std::size_t i = 0; i < std::strlen(needle.text); i++) {
+    switch (needle.text[i]) {
+      case '\\':
+        // The next character is escaped, so it can't possibly be the
+        // metacharacter '('.  Skip it.
+        i++;
+        break;
+      case '(':
+        // That's our metacharacter.
+        result.n_matches++;
+        break;
+      default:
+        // Nothing interesting.
+        break;
+    }
+  }
+  // Allocate to store the match information.  Allocate one more slot than we
+  // need, because the regex engine puts information about the entire match in
+  // the first slot.
+  result.matches =
+      reinterpret_cast<regmatch_t*>(
+          uw_malloc(context, (result.n_matches + 1) * sizeof(regmatch_t)));
+  // Execute the regex.
+  switch (regexec(needle.compiled, haystack, result.n_matches + 1,
+                  result.matches, 0)) {
+    case 0:
+      // A match occurred.
+      result.succeeded = 1;
+      // Bump the matches array to skip information about the entire match.
+      result.matches++;
+      break;
+    case REG_NOMATCH:
+      // No match occurred.
+      result.succeeded = 0;
+      result.n_matches = 0;
+      result.matches = nullptr;
+      break;
+    case REG_ESPACE:
+      // We ran out of memory.
+      uw_error(context, BOUNDED_RETRY, "regex: could not allocate");
+    default:
+      // Some unknown error occurred.
+      uw_error(context, FATAL, "regex: could not execute regular expression");
+  }
+  return result;
+}