[Libvir] [PATCH] Enhance virBuffer code

[Richard W.M. Jones]

Jim Meyering wrote:
> "Richard W.M. Jones" <rjones at redhat.com> wrote:
> 
>> Jim Meyering wrote:
>>> "Richard W.M. Jones" <rjones at redhat.com> wrote:
>>>> Jim Meyering wrote:
>>>>> What do you think of using this?
>>>>>
>>>>>   isascii (*p) && isalnum (*p)
>>>> I'm not sure I'm qualified to say what this does on EBCDIC, but quite
>>>> likely lots of other code breaks there too anyway.  This is nicely
>>>> self-documenting anyway.
>>> As Daniel suggested, isalnum is locale-sensitive.
>>> If there's a locale with an alphabetic byte that is outside
>>> the logical a-zA-Z range, yet still within 0..127, then the above
>>> expression will give a false-positive for that byte.
>>>
>>> I've been inclined to stop worrying about EBCDIC for years, but a quick
>>> search on the web finds that people are still stuck using it, and do
>>> report bugs in ASCII-assuming code.
>>>
>>> This is why autoconf goes to the trouble of doing this:
>>>   tr abcdefghijklmnopqrstuvwxyz ABCDEFGHIJKLMNOPQRSTUVWXYZ
>>> not this:
>>>   tr a-z A-Z
>>> to convert to upper case.
>> Another factor to consider here is that it doesn't matter if this
>> function over-escapes, but it does matter if the function
>> under-escapes. That is to say, it could escape every character as a
>> %xx hex code, which would be ugly and slightly inefficient but not
>> wrong.
> 
> IMHO, if you don't use the all-enumerating switch-based code that Daniel
> objects to, it'd be good to document (in both loops) that the test is
> inaccurate with EBCDIC, and explain why it's ok to get false positives.
> 
> Without comments, people might be tempted to use a similar test in a
> context where the differences matter.

OK, how about this?

Rich.

+    for (p = str; *p; ++p) {
+        /* Want to escape only A-Z and 0-9.  This may not work on 
EBCDIC. */
+        if (isascii (*p) && isalnum (*p))
+            grow_size++;
+        else
+            grow_size += 3; /* %ab */
+    }
+
+    if (virBufferGrow (buf, grow_size) == -1)
+        return -1;
+
+    for (p = str; *p; ++p) {
+       /* See comment above. */
+        if (isascii (*p) && isalnum (*p))
+            buf->content[buf->use++] = *p;
+        else {
+            uc = (unsigned char) *p;
+            buf->content[buf->use++] = '%';
+            buf->content[buf->use++] = hex[uc >> 4];
+            buf->content[buf->use++] = hex[uc & 0xf];
+        }
+    }


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