More work on windows locking. Fix some problems with unix locking. There

is still an assertion failure on windows locking in attach2.test. (CVS 1539)

FossilOrigin-Name: 0c2d169cf3c0f36972015c952a2b46cb9a333881

src/os.h

@@ -90,6 +90,57 @@
 #define PENDING_LOCK    3
 #define EXCLUSIVE_LOCK  4
 
+/*
+** Windows file locking notes:
+**
+** We cannot use LockFileEx() or UnlockFileEx() on Win95/98/ME because
+** those functions are not available.  So we use only LockFile() and
+** UnlockFile().
+**
+** LockFile() prevents not just writing but also reading by other processes.
+** (This is a design error on the part of Windows, but there is nothing
+** we can do about that.)  So the region used for locking is at the
+** end of the file where it is unlikely to ever interfere with an
+** actual read attempt.
+**
+** A SHARED_LOCK is obtained by locking a single randomly-chosen 
+** byte out of a specific range of bytes. The lock byte is obtained at 
+** random so two separate readers can probably access the file at the 
+** same time, unless they are unlucky and choose the same lock byte.
+** An EXCLUSIVE_LOCK is obtained by locking all bytes in the range.
+** There can only be one writer.  A RESERVED_LOCK is obtained by locking
+** a single byte of the file that is designated as the reserved lock byte.
+** A PENDING_LOCK is obtained by locking a designated byte different from
+** the RESERVED_LOCK byte.
+**
+** On WinNT/2K/XP systems, LockFileEx() and UnlockFileEx() are available,
+** which means we can use reader/writer locks.  When reader/writer locks
+** are used, the lock is placed on the same range of bytes that is used
+** for probabilistic locking in Win95/98/ME.  Hence, the locking scheme
+** will support two or more Win95 readers or two or more WinNT readers.
+** But a single Win95 reader will lock out all WinNT readers and a single
+** WinNT reader will lock out all other Win95 readers.
+**
+** The following #defines specify the range of bytes used for locking.
+** SHARED_SIZE is the number of bytes available in the pool from which
+** a random byte is selected for a shared lock.  The pool of bytes for
+** shared locks begins at SHARED_FIRST. 
+**
+** These #defines are available in os.h so that Unix can use the same
+** byte ranges for locking.  This leaves open the possiblity of having
+** clients on win95, winNT, and unix all talking to the same shared file
+** and all locking correctly.
+**
+** Locking in windows is manditory.  For this reason, we cannot store
+** actual data in the bytes used for locking.  The pager never allocates
+** the pages involved in locking therefore.
+*/
+#define SHARED_SIZE       10238
+#define SHARED_FIRST      (0x3fffffff - (SHARED_SIZE - 1))
+#define RESERVED_BYTE     (SHARED_FIRST - 1)
+#define PENDING_BYTE      (RESERVED_BYTE - 1)
+
+
 int sqlite3OsDelete(const char*);
 int sqlite3OsFileExists(const char*);
 int sqliteOsFileRename(const char*, const char*);