Simplify RA-svn's string marshalling.

This is a follow-up to r1028352 which merged r985606 and r1028092 from the
performance branch.  That change used separate functions to handle short and
long strings.  This change instead detects and fails on strings that are too
long to be valid.

* subversion/libsvn_ra_svn/marshal.c
  (SUSPICIOUSLY_HUGE_STRING_SIZE_THRESHOLD): Re-write doc string.
  (read_long_string): Remove.
  (read_string): Fail if the received string claims to be impossibly long.
--This line, and those below, will be ignored--

Index: subversion/libsvn_ra_svn/marshal.c
===================================================================
--- subversion/libsvn_ra_svn/marshal.c	(revision 1132599)
+++ subversion/libsvn_ra_svn/marshal.c	(working copy)
@@ -46,10 +46,10 @@
 
 #define svn_iswhitespace(c) ((c) == ' ' || (c) == '\n')
 
-/* If we receive data that *claims* to be followed by a very long string,
- * we should not trust that claim right away. But everything up to 1 MB
- * should be too small to be instrumental for a DOS attack. */
-
+/* The maximum legitimate size of a received string.  This just needs to be
+ * large enough to contain an svndiff window and small enough not to be
+ * instrumental in a DOS attack.
+ * ### Can we base this more concretely on SVN_DELTA_WINDOW_SIZE? */
 #define SUSPICIOUSLY_HUGE_STRING_SIZE_THRESHOLD (0x100000)
 
 /* --- CONNECTION INITIALIZATION --- */
@@ -563,36 +563,6 @@
 
 /* --- READING DATA ITEMS --- */
 
-/* Read LEN bytes from CONN into a supposedly empty STRINGBUF.
- * POOL will be used for temporary allocations. */
-static svn_error_t *
-read_long_string(svn_ra_svn_conn_t *conn, apr_pool_t *pool,
-                 svn_stringbuf_t *stringbuf, apr_uint64_t len)
-{
-  char readbuf[4096];
-  apr_size_t readbuf_len;
-
-  /* We can't store strings longer than the maximum size of apr_size_t,
-   * so check for wrapping */
-  if (((apr_size_t) len) < len)
-    return svn_error_create(SVN_ERR_RA_SVN_MALFORMED_DATA, NULL,
-                            _("String length larger than maximum"));
-
-  while (len)
-    {
-      readbuf_len = len > sizeof(readbuf) ? sizeof(readbuf) : (apr_size_t)len;
-
-      SVN_ERR(readbuf_read(conn, pool, readbuf, readbuf_len));
-      /* Read into a stringbuf_t to so we don't allow the sender to allocate
-       * an arbitrary amount of memory without actually sending us that much
-       * data */
-      svn_stringbuf_appendbytes(stringbuf, readbuf, readbuf_len);
-      len -= readbuf_len;
-    }
-
-  return SVN_NO_ERROR;
-}
-
 /* Read LEN bytes from CONN into already-allocated structure ITEM.
  * Afterwards, *ITEM is of type 'SVN_RA_SVN_STRING', and its string
  * data is allocated in POOL. */
@@ -601,27 +571,15 @@
 {
   svn_stringbuf_t *stringbuf;
 
-  /* We should not use large strings in our protocol. However, we may
-   * receive a claim that a very long string is going to follow. In that
-   * case, we start small and wait for all that data to actually show up.
-   * This does not fully prevent DOS attacs but makes them harder (you
-   * have to actually send gigabytes of data).
-   */
+  /* We should not use large strings in our protocol. Fail if we
+   * receive a claim that an impossibly long string is going to follow,
+   * in order to prevent a simple kind of DOS attack. */
   if (len > SUSPICIOUSLY_HUGE_STRING_SIZE_THRESHOLD)
-    {
-      /* This string might take a large amount of memory. Don't allocate
-       * the whole buffer at once, so to prevent OOM issues by corrupted
-       * network data.
-       */
-      stringbuf = svn_stringbuf_create("", pool);
-      SVN_ERR(read_long_string(conn, pool, stringbuf, len));
-    }
-  else
-    {
+    return svn_error_create(SVN_ERR_RA_SVN_MALFORMED_DATA, NULL,
+                            _("Received string is impossibly long"));
+
       /* This is a reasonably sized string. So, provide a buffer large
-       * enough to prevent re-allocation as long as the data transmission
-       * is not flawed.
-       */
+   * enough not to need re-allocation. */
       stringbuf = svn_stringbuf_create_ensure((apr_size_t)len, pool);
 
       /* Read the string data directly into the string structure.
@@ -637,7 +595,6 @@
           stringbuf->data[stringbuf->len] = '\0';
           len -= readbuf_len;
         }
-    }
 
   /* Return the string properly wrapped into an RA_SVN item. */
   item->kind = SVN_RA_SVN_STRING;