strbuf: Free memory when it's not longer needed.

While this might generate some churn in some situations, care has been taken
to avoid this as much as possible while providing a (hopefully) good balance
between memory usage and the need to reallocate memory.

diff --git a/src/utils/strbuf.c b/src/utils/strbuf.c
index 822f01928c4437b40bb4d621f905cf0b649cea6a..6c881be18e4975517f4c5080747e5d6f83a183f1 100644 (file)
--- a/src/utils/strbuf.c
+++ b/src/utils/strbuf.c
@@ -36,6 +36,14 @@
 
 #include <unistd.h>
 
+/* free memory if most of the buffer is unused */
+#define CHECK_SHRINK(strbuf) \
+       do { \
+               if ((strbuf)->pos < (strbuf)->size / 3) \
+                       /* don't free all memory to avoid churn */ \
+                       strbuf_resize((strbuf), 2 * (strbuf)->pos); \
+       } while (0)
+
 /*
  * private data structures
  */
@@ -55,14 +63,17 @@ strbuf_resize(sdb_strbuf_t *strbuf, size_t new_size)
 {
        char *tmp;
 
-       if (new_size <= strbuf->size)
-               return 0;
+       if (new_size <= strbuf->pos)
+               return -1;
 
        tmp = realloc(strbuf->string, new_size);
        if (! tmp)
                return -1;
 
-       strbuf->string = tmp;
+       if (new_size)
+               strbuf->string = tmp;
+       else
+               strbuf->string = NULL;
        strbuf->size = new_size;
        return 0;
 } /* strbuf_resize */
@@ -144,7 +155,7 @@ sdb_strbuf_vappend(sdb_strbuf_t *strbuf, const char *fmt, va_list ap)
 
        /* 'status' does not include nul-byte */
        if ((size_t)status >= strbuf->size - strbuf->pos - 1) {
-               if (strbuf_resize(strbuf, strbuf->size + (size_t)status)) {
+               if (strbuf_resize(strbuf, strbuf->size + (size_t)status + 1)) {
                        va_end(aq);
                        return -1;
                }
@@ -157,6 +168,11 @@ sdb_strbuf_vappend(sdb_strbuf_t *strbuf, const char *fmt, va_list ap)
                strbuf->pos += (size_t)status;
 
        va_end(aq);
+
+       /* even though this function always appends to the existing buffer, the
+        * size might have previously been reset */
+       CHECK_SHRINK(strbuf);
+
        return (ssize_t)status;
 } /* sdb_strbuf_vappend */
 
@@ -227,6 +243,10 @@ sdb_strbuf_memappend(sdb_strbuf_t *strbuf, const void *data, size_t n)
        strbuf->pos += n;
        strbuf->string[strbuf->pos] = '\0';
 
+       /* even though this function always appends to the existing buffer, the
+        * size might have previously been reset */
+       CHECK_SHRINK(strbuf);
+
        return (ssize_t)n;
 } /* sdb_strbuf_memappend */
 
@@ -252,8 +272,9 @@ sdb_strbuf_read(sdb_strbuf_t *strbuf, int fd, size_t n)
        if (! strbuf)
                return -1;
 
-       if (strbuf_resize(strbuf, strbuf->pos + n + 1))
-               return -1;
+       if (strbuf->pos + n + 1 >= strbuf->size)
+               if (strbuf_resize(strbuf, strbuf->pos + n + 1))
+                       return -1;
 
        ret = read(fd, strbuf->string + strbuf->pos, n);
        if (ret > 0)
@@ -309,6 +330,8 @@ sdb_strbuf_skip(sdb_strbuf_t *strbuf, size_t offset, size_t n)
        memmove(start, start + n, len - n);
        strbuf->pos -= n;
        strbuf->string[strbuf->pos] = '\0';
+
+       /* don't resize now but wait for the next write to avoid churn */
 } /* sdb_strbuf_skip */
 
 void
@@ -319,6 +342,8 @@ sdb_strbuf_clear(sdb_strbuf_t *strbuf)
 
        strbuf->string[0] = '\0';
        strbuf->pos = 0;
+
+       /* don't resize now but wait for the next write to avoid churn */
 } /* sdb_strbuf_clear */
 
 const char *