Add some comments regarding file-locks to log.c.
FossilOrigin-Name: 9d51c3b754f0b94fea5ef3d669ad583b93b2b024
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C Do\snot\ssync\sany\sfiles\sin\swal\smode\sif\s"PRAGMA\ssynchronous=off"\sis\sset.\sIf\sfiles\sare\ssynced,\spass\seither\sSQLITE_SYNC_FULL\sor\sSQLITE_SYNC_NORMAL\sto\sthe\sxSync()\scallback\sas\sconfigured\sby\s"PRAGMA\sfullfsync".
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D 2010-04-17T17:34:42
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C Add\ssome\scomments\sregarding\sfile-locks\sto\slog.c.
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D 2010-04-17T18:50:28
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F Makefile.arm-wince-mingw32ce-gcc fcd5e9cd67fe88836360bb4f9ef4cb7f8e2fb5a0
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F Makefile.in 4f2f967b7e58a35bb74fb7ec8ae90e0f4ca7868b
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F Makefile.linux-gcc d53183f4aa6a9192d249731c90dbdffbd2c68654
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@ -131,7 +131,7 @@ F src/journal.c b0ea6b70b532961118ab70301c00a33089f9315c
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F src/legacy.c a199d7683d60cef73089e892409113e69c23a99f
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F src/lempar.c 7f026423f4d71d989e719a743f98a1cbd4e6d99e
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F src/loadext.c 1c7a61ce1281041f437333f366a96aa0d29bb581
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F src/log.c 8bcd490de703a12563c939a59cb4e187a0c67fe3
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F src/log.c fd6631f740b0cf2c8720477a036ffee8ca705c92
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F src/log.h 3cfec02156f5db3d233352897a47601930a29955
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F src/main.c c0e7192bad5b90544508b241eb2487ac661de890
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F src/malloc.c a08f16d134f0bfab6b20c3cd142ebf3e58235a6a
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@ -805,7 +805,7 @@ F tool/speedtest2.tcl ee2149167303ba8e95af97873c575c3e0fab58ff
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F tool/speedtest8.c 2902c46588c40b55661e471d7a86e4dd71a18224
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F tool/speedtest8inst1.c 293327bc76823f473684d589a8160bde1f52c14e
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F tool/vdbe-compress.tcl d70ea6d8a19e3571d7ab8c9b75cba86d1173ff0f
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P 43463970f5885fb116588695146f2a56cb22804a
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R 0e49b7b76893a9b984350c6fdab0ea9a
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P 0ae91b0008b242a47385fc1f295c6b645483ee22
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R a205bea3ab4fa24118ce751726c3c719
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U dan
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Z ad3f9c0ef28635a13cd67b8f240d7d22
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Z c979f9f88d66076be76b02e7ab3fa216
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@ -1 +1 @@
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0ae91b0008b242a47385fc1f295c6b645483ee22
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9d51c3b754f0b94fea5ef3d669ad583b93b2b024
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91
src/log.c
91
src/log.c
@ -103,6 +103,93 @@ struct LogSummary {
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u32 *aData; /* File body */
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};
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/*
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** This module uses three different types of file-locks. All are taken
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** on the log-summary file. The three types of locks are as follows:
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**
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** MUTEX: The MUTEX lock is used as a robust inter-process mutex. It
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** is held while the log-summary header is modified, and
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** sometimes when it is read. It is also held while a new client
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** obtains the DMH lock (see below), and while log recovery is
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** being run.
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**
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** DMH: The DMH (Dead Mans Hand mechanism) lock is used to ensure
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** that log-recovery is always run following a system restart.
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** When it first opens a log-summary file, a process takes a
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** SHARED lock on the DMH region. This lock is not released until
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** the log-summary file is closed.
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**
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** The process then attempts to upgrade to an EXCLUSIVE lock. If
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** successful, then the contents of the log-summary file are deemed
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** suspect and the log-summary header zeroed. This forces the
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** first process that reads the log-summary file to run log
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** recovery. After zeroing the log-summary header, the process
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** downgrades to a SHARED lock on the DMH region.
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**
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** If the attempt to obtain the EXCLUSIVE lock fails, then the
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** process concludes that some other process is already using the
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** log-summary file, and it can therefore be trusted.
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**
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** The procedure described in the previous three paragraphs (taking
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** a SHARED lock and then upgrading to an EXCLUSIVE lock to check
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** if the process is the only one to have an open connection to the
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** log file) is protected by holding the MUTEX lock. This avoids the
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** race condition wherein the first two clients connect almost
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** simultaneously following a system restart and each prevents
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** the other from obtaining the EXCLUSIVE lock.
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**
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**
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** REGION: There are 4 different region locks, regions A, B, C and D.
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** Various EXCLUSIVE and SHARED locks on these regions are obtained
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** when a client reads, writes or checkpoints the database.
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**
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** To obtain a reader lock:
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**
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** 1. Attempt a SHARED lock on regions A and B.
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** 2. If step 1 is successful, drop the lock on region B. Or, if
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** it is unsuccessful, attempt a SHARED lock on region D.
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** 3. Repeat the above until the lock attempt in step 1 or 2 is
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** successful.
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**
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** The reader lock is released when the read transaction is finished.
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**
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** To obtain a writer lock:
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**
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** 1. Take (wait for) an EXCLUSIVE lock on regions C and D.
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**
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** The locks are released after the write transaction is finished
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** and, if any frames were committed to the log, the log-summary
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** file updated.
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**
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** To obtain a checkpointer lock:
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**
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** 1. Take (wait for) an EXCLUSIVE lock on regions B and C.
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** 2. Take (wait for) an EXCLUSIVE lock on region A.
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**
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** Step 1 waits until any existing writer has finished. And forces
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** all new readers to become "region D" readers.
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**
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** Step 2 causes the checkpointer to wait until all existing region A
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** readers have finished their transactions. Once the exclusive lock
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** on region A has been obtained, only "region D" readers exist.
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** These readers are operating on the snapshot at the head of the
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** log. As such, the log can be safely copied into the database file
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** without interfering with the readers.
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**
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** Once the checkpoint has finished and the log-summary header
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** updated (to indicate the log contents can now be ignored), all
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** locks are released.
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**
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** However, there may still exist region D readers using data in
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** the body of the log file, so the log file itself cannot be
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** truncated or overwritten until all region D readers have finished.
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** That requirement is satisfied, because writers (the clients that
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** write to the log file) require an exclusive lock on region D.
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** Which they cannot get until all region D readers have finished.
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*/
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#define LOG_LOCK_MUTEX 12
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#define LOG_LOCK_DMH 13
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#define LOG_LOCK_REGION 14
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/*
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** The four lockable regions associated with each log-summary. A connection
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@ -115,10 +202,6 @@ struct LogSummary {
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#define LOG_REGION_C 0x04
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#define LOG_REGION_D 0x08
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#define LOG_LOCK_MUTEX 12
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#define LOG_LOCK_DMH 13
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#define LOG_LOCK_REGION 14
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/*
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** A single instance of this structure is allocated as part of each
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** connection to a database log. All structures associated with the
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