Support maximum field width for string access.log fields.

Some standard command-line and some log processing tools have trouble handling
URLs or other logged fields exceeding 8KB in length. Moreover, Squid violates
its own log line format and truncates the entire log line if, for example, the
URL is 8KB long. By supporting .precision format argument, we allow the
administrator to specify logged URL size and avoid these problems.

Limiting logged field width has no effect on traffic on the wire.

TODO: The name comes from the printf(3) "precision" format part. It may be a
good idea to rename our "precision" into max_width or similar, especially if
we do not support floating point precision logging.

TODO: Old code used chars to store user-configured field width and precision.
That does not work for URLs, headers, and other entries longer than 256
characters. This patch changes the storage type to int. The code should
probably be polished further to remove unsigned->signed conversions.

=== modified file 'src/cf.data.pre'
--- src/cf.data.pre	2010-01-02 04:32:46 +0000
+++ src/cf.data.pre	2010-01-20 17:15:59 +0000
@@ -2462,7 +2462,7 @@
 	modifiers are usually not needed, but can be specified if an explicit
 	output format is desired.
 
-		% ["|[|'|#] [-] [[0]width] [{argument}] formatcode
+		% ["|[|'|#] [-] [[0]width[.precision]] [{argument}] formatcode
 
 		"	output in quoted string format
 		[	output in squid text log format as used by log_mime_hdrs
@@ -2470,8 +2470,10 @@
 		'	output as-is
 
 		-	left aligned
-		width	field width. If starting with 0 the
-			output is zero padded
+		width	minimum field width. 
+			If starting with 0 the output is zero padded
+		precision	maximum field width for string values.
+			Longer string values are truncated.
 		{arg}	argument such as header name etc
 
 	Format codes:

=== modified file 'src/log/access_log.cc'
--- src/log/access_log.cc	2009-12-22 01:12:53 +0000
+++ src/log/access_log.cc	2010-01-20 17:15:59 +0000
@@ -466,8 +466,8 @@
         } header;
         char *timespec;
     } data;
-    unsigned char width;
-    unsigned char precision;
+    unsigned int width;
+    unsigned int precision;
     enum log_quote quote;
     unsigned int left:1;
     unsigned int space:1;
@@ -1213,13 +1213,15 @@
                 }
             }
 
-            if (fmt->width) {
-                if (fmt->left)
-                    mb.Printf("%-*s", (int) fmt->width, out);
-                else
-                    mb.Printf("%*s", (int) fmt->width, out);
-            } else
-                mb.append(out, strlen(out));
+            // enforce width limits if configured
+            const int minWidth = fmt->width ? static_cast<int>(fmt->width) : 0;
+            const int maxWidth = (fmt->precision && !doint && !dooff) ?
+                static_cast<int>(fmt->precision) : strlen(out);
+
+            if (fmt->left)
+                mb.Printf("%-*.*s", minWidth, maxWidth, out);
+            else
+                mb.Printf("%*.*s", minWidth, maxWidth, out);
         } else {
             mb.append("-", 1);
         }