From: Martin Mares <mj@ucw.cz>
Date: Wed, 13 Sep 2006 19:38:05 +0000 (+0200)
Subject: Atomic fastbufs: the first sketch of the interface.
X-Git-Tag: holmes-import~512^2~7
X-Git-Url: http://mj.ucw.cz/gitweb/?a=commitdiff_plain;h=9dc6f017d0b8c992d2df15a51cf96fc431260df3;p=libucw.git

Atomic fastbufs: the first sketch of the interface.
---

diff --git a/lib/Makefile b/lib/Makefile
index 2a554870..775c7de6 100644
--- a/lib/Makefile
+++ b/lib/Makefile
@@ -15,7 +15,7 @@ LIBUCW_MODS= \
 	ipaccess \
 	profile \
 	fastbuf ff-binary ff-string ff-printf ff-utf8 \
-	fb-file carefulio fb-mem fb-temp fb-mmap fb-limfd fb-buffer fb-grow \
+	fb-file carefulio fb-mem fb-temp fb-mmap fb-limfd fb-buffer fb-grow fb-atomic \
 	str_ctype str_upper str_lower unicode-utf8 stkstring \
 	wildmatch wordsplit ctmatch patimatch patmatch regex \
 	prime primetable random timer randomkey \
diff --git a/lib/fastbuf.h b/lib/fastbuf.h
index 6dbacce5..2d1c1eb3 100644
--- a/lib/fastbuf.h
+++ b/lib/fastbuf.h
@@ -118,6 +118,11 @@ struct fastbuf *fbgrow_create(unsigned basic_size);
 void fbgrow_reset(struct fastbuf *b);			/* Reset stream and prepare for writing */
 void fbgrow_rewind(struct fastbuf *b);			/* Prepare for reading */
 
+/* FastO with atomic writes for multi-threaded programs */
+
+struct fastbuf *fbatomic_create(byte *name, struct fastbuf *master, uns bufsize, int record_len);
+void fbatomic_checkpoint(struct fastbuf *b);
+
 /* Configuring stream parameters */
 
 int bconfig(struct fastbuf *f, uns type, int data);
diff --git a/lib/fb-atomic.c b/lib/fb-atomic.c
new file mode 100644
index 00000000..29e7c303
--- /dev/null
+++ b/lib/fb-atomic.c
@@ -0,0 +1,50 @@
+/*
+ *	UCW Library -- Atomic Buffered Write to Files
+ *
+ *	(c) 2006 Martin Mares <mj@ucw.cz>
+ *
+ *	This software may be freely distributed and used according to the terms
+ *	of the GNU Lesser General Public License.
+ */
+
+/*
+ *	This fastbuf backend is intended for cases where several threads
+ *	of a single program append records to a single file and while the
+ *	record can mix in an arbitrary way, the bytes inside a single
+ *	record must remain uninterrupted.
+ *
+ *	In case of files with fixed record size, we just allocate the
+ *	buffer to hold a whole number of records and take advantage
+ *	of the atomicity of the write() system call.
+ *
+ *	With variable-sized records, we need another solution: when
+ *	writing a record, we keep the fastbuf in a locked state, which
+ *	prevents buffer flushing (and if the buffer becomes full, we extend it),
+ *	and we wait for an explicit commit operation which write()s the buffer
+ *	if the free space in the buffer falls below the expected maximum record
+ *	length.
+ *
+ *	fbatomic_create() is called with the following parameters:
+ *	    name - name of the file to open
+ *	    master - fbatomic for the master thread or NULL if it's the first open
+ *	    bufsize - initial buffer size
+ *	    record_len - record length for fixed-size records;
+ *		or -(expected maximum record length) for variable-sized ones.
+ */
+
+#include "lib/lib.h"
+#include "lib/fastbuf.h"
+#include "lib/lfs.h"
+
+#include <string.h>
+#include <fcntl.h>
+#include <unistd.h>
+
+struct fb_atomic {
+  struct fastbuf fb;
+  int fd;				/* File descriptor */
+  uns locked;
+  uns expected_max_len;
+  byte *expected_max_bptr;
+};
+#define FB_ATOMIC(f) ((struct fb_atomic *)(f)->is_fastbuf)