qapi: Support (subset of) \u escapes in strings

The handling of \ inside QAPI strings was less than ideal, and
really only worked JSON's \/, \\, \", and our extension of \'
(an obvious extension, when you realize we use '' instead of ""
for strings). For other things, like '\n', it resulted in a
literal 'n' instead of a newline.

Of course, at the moment, we really have no use for escaped
characters, as QAPI has to map to C identifiers, and we currently
support ASCII only for that. But down the road, we may add
support for default values for string parameters to a command
or struct; if that happens, it would be nice to correctly support
all JSON escape sequences, such as \n or \uXXXX. This gets us
closer, by supporting Unicode escapes in the ASCII range.

Since JSON does not require \OCTAL or \xXX escapes, and our QMP
implementation does not understand them either, I intentionally
reject it here, but it would be an easy addition if we desired it.
Likewise, intentionally refusing the NUL byte means we don't have
to worry about C strings being shorter than the qapi input.

Backports commit a7f5966b297330f6492020019544ae87c45d699b from qemu

qemu/scripts/qapi.py

@@ -173,7 +173,41 @@ class QAPISchema:
                         raise QAPISchemaError(self,
                                               'Missing terminating "\'"')
                     if esc:
+                        if ch == 'b':
+                            string += '\b'
+                        elif ch == 'f':
+                            string += '\f'
+                        elif ch == 'n':
+                            string += '\n'
+                        elif ch == 'r':
+                            string += '\r'
+                        elif ch == 't':
+                            string += '\t'
+                        elif ch == 'u':
+                            value = 0
+                            for x in range(0, 4):
+                                ch = self.src[self.cursor]
+                                self.cursor += 1
+                                if ch not in "0123456789abcdefABCDEF":
+                                    raise QAPISchemaError(self,
+                                                          '\\u escape needs 4 '
+                                                          'hex digits')
+                                value = (value << 4) + int(ch, 16)
+                            # If Python 2 and 3 didn't disagree so much on
+                            # how to handle Unicode, then we could allow
+                            # Unicode string defaults.  But most of QAPI is
+                            # ASCII-only, so we aren't losing much for now.
+                            if not value or value > 0x7f:
+                                raise QAPISchemaError(self,
+                                                      'For now, \\u escape '
+                                                      'only supports non-zero '
+                                                      'values up to \\u007f')
+                            string += chr(value)
+                        elif ch in "\\/'\"":
                             string += ch
+                        else:
+                            raise QAPISchemaError(self,
+                                                  "Unknown escape \\%s" %ch)
                         esc = False
                     elif ch == "\\":
                         esc = True