Improve escaping in configuration files

Replace pairs of backslashes before '@' or '\@' with singles (allows
escaping the escape character). Do not consume next '@' after '\@'.

mesonbuild/mesonlib.py

@@ -428,19 +428,28 @@ def do_replacement(regex, line, confdata):
     missing_variables = set()
 
     def variable_replace(match):
-        varname = match.group(1)
-        if varname in confdata:
-            (var, desc) = confdata.get(varname)
-            if isinstance(var, str):
-                pass
-            elif isinstance(var, int):
-                var = str(var)
-            else:
-                raise RuntimeError('Tried to replace a variable with something other than a string or int.')
+        # Pairs of escape characters before '@' or '\@'
+        if match.group(0).endswith('\\'):
+            num_escapes = match.end(0) - match.start(0)
+            return '\\' * (num_escapes // 2)
+        # Single escape character and '@'
+        elif match.group(0) == '\\@':
+            return '@'
+        # Template variable to be replaced
         else:
-            missing_variables.add(varname)
-            var = ''
-        return var
+            varname = match.group(1)
+            if varname in confdata:
+                (var, desc) = confdata.get(varname)
+                if isinstance(var, str):
+                    pass
+                elif isinstance(var, int):
+                    var = str(var)
+                else:
+                    raise RuntimeError('Tried to replace a variable with something other than a string or int.')
+            else:
+                missing_variables.add(varname)
+                var = ''
+            return var
     return re.sub(regex, variable_replace, line), missing_variables
 
 def do_mesondefine(line, confdata):
@@ -473,7 +482,7 @@ def do_conf_file(src, dst, confdata):
         raise MesonException('Could not read input file %s: %s' % (src, str(e)))
     # Only allow (a-z, A-Z, 0-9, _, -) as valid characters for a define
     # Also allow escaping '@' with '\@'
-    regex = re.compile(r'(?<!\\)@([-a-zA-Z0-9_]+)@')
+    regex = re.compile(r'(?:\\\\)+(?=\\?@)|\\@|@([-a-zA-Z0-9_]+)@')
     result = []
     missing_variables = set()
     for line in data:

test cases/common/16 configure file/config6.h.in

@@ -0,0 +1,19 @@
+/* No escape */
+#define MESSAGE1 "@var1@"
+
+/* Single escape means no replace */
+#define MESSAGE2 "\@var1@"
+
+/* Replace pairs of escapes before '@' or '\@' with escape characters
+ * (note we have to double number of pairs due to C string escaping)
+ */
+#define MESSAGE3 "\\\\@var1@"
+
+/* Pairs of escapes and then single escape to avoid replace */
+#define MESSAGE4 "\\\\\@var1@"
+
+/* Check escaped variable does not overlap following variable */
+#define MESSAGE5 "\@var1@var2@"
+
+/* Check escape character outside variables */
+#define MESSAGE6 "\\ @ \@ \\\\@ \\\\\@"

test cases/common/16 configure file/meson.build

@@ -120,3 +120,14 @@ configure_file(
   configuration : conf5
 )
 test('test5', executable('prog5', 'prog5.c'))
+
+# Test escaping
+conf6 = configuration_data()
+conf6.set('var1', 'foo')
+conf6.set('var2', 'bar')
+configure_file(
+  input : 'config6.h.in',
+  output : '@BASENAME@',
+  configuration : conf6
+)
+test('test6', executable('prog6', 'prog6.c'))

test cases/common/16 configure file/prog6.c

@@ -0,0 +1,11 @@
+#include <string.h>
+#include <config6.h>
+
+int main(int argc, char **argv) {
+    return strcmp(MESSAGE1, "foo")
+        || strcmp(MESSAGE2, "@var1@")
+        || strcmp(MESSAGE3, "\\foo")
+        || strcmp(MESSAGE4, "\\@var1@")
+        || strcmp(MESSAGE5, "@var1bar")
+        || strcmp(MESSAGE6, "\\ @ @ \\@ \\@");
+}