Clarify some comments describing the WAL index file. No changes to code.

FossilOrigin-Name: 3be3aad9ecbe33060cfa9c6059b9206ed221d1fd72a69c355a9387f9f4e075e7

manifest

@@ -1,5 +1,5 @@
-C Improve\sthe\sperformance\sof\sthe\sLIKE\soperator\sby\susing\sstrcspn()\sto\said\nwildcard\smatching.
-D 2017-10-30T18:49:11.595
+C Clarify\ssome\scomments\sdescribing\sthe\sWAL\sindex\sfile.\s\sNo\schanges\sto\scode.
+D 2017-10-30T20:44:36.285
 F Makefile.in e016061b23e60ac9ec27c65cb577292b6bde0307ca55abd874ab3487b3b1beb2
 F Makefile.linux-gcc 7bc79876b875010e8c8f9502eb935ca92aa3c434
 F Makefile.msc 37740aba9c4bb359c627eadccf1cfd7be4f5f847078723777ea7763969e533b1
@@ -543,7 +543,7 @@ F src/vdbesort.c 731a09e5cb9e96b70c394c1b7cf3860fbe84acca7682e178615eb941a3a0ef2
 F src/vdbetrace.c 48e11ebe040c6b41d146abed2602e3d00d621d7ebe4eb29b0a0f1617fd3c2f6c
 F src/vtab.c 0e4885495172e1bdf54b12cce23b395ac74ef5729031f15e1bc1e3e6b360ed1a
 F src/vxworks.h d2988f4e5a61a4dfe82c6524dd3d6e4f2ce3cdb9
-F src/wal.c 839db09792fead5052bb35e533fa485e134913d547d05b5f42e537b73e63f07a
+F src/wal.c 712947ad0e37ccb852c631527fd708cc0dea8b6fd503c8fd070370aae37bbe56
 F src/wal.h 8de5d2d3de0956d6f6cb48c83a4012d5f227b8fe940f3a349a4b7e85ebcb492a
 F src/walker.c 3ccfa8637f95355bff61144e01a615b8ef26f79c312880848da73f03367da1e6
 F src/where.c d8c6d690c4b11f30211de073011fe19352364a6303ae053f45cb66f9576ba8a9
@@ -1666,7 +1666,7 @@ F vsixtest/vsixtest.tcl 6a9a6ab600c25a91a7acc6293828957a386a8a93
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P 457eedfac0ff652912c4cfb89682acb3d2e372ef00745139adf8492a0b61fa09
-R b136c0336cd9a2670f5306d89a5f1d34
+P 37284d4e8f501a37c582aa899419a3dfe0932e2a8e2ef2fdf59addd1d3cdacb4
+R b73d22fe388dfec64db639a4eb4f982f
 U drh
-Z 1ff3723c7287a54f5348d686239a8395
+Z b8d196e6ebc579e3254aa8271770e0e4

manifest.uuid

@@ -1 +1 @@
-37284d4e8f501a37c582aa899419a3dfe0932e2a8e2ef2fdf59addd1d3cdacb4
+3be3aad9ecbe33060cfa9c6059b9206ed221d1fd72a69c355a9387f9f4e075e7

src/wal.c

@@ -132,6 +132,10 @@
 ** on a network filesystem.  All users of the database must be able to
 ** share memory.
 **
+** In the default unix and windows implementation, the wal-index is a mmapped
+** file whose name is the database name with a "-shm" suffix added.  For that
+** reason, the wal-index is sometimes called the "shm" file.
+**
 ** The wal-index is transient.  After a crash, the wal-index can (and should
 ** be) reconstructed from the original WAL file.  In fact, the VFS is required
 ** to either truncate or zero the header of the wal-index when the last
@@ -271,9 +275,18 @@ int sqlite3WalTrace = 0;
 #define WALINDEX_MAX_VERSION 3007000
 
 /*
-** Indices of various locking bytes.   WAL_NREADER is the number
+** Index numbers for various locking bytes.   WAL_NREADER is the number
 ** of available reader locks and should be at least 3.  The default
 ** is SQLITE_SHM_NLOCK==8 and  WAL_NREADER==5.
+**
+** Technically, the various VFSes are free to implement these locks however
+** they see fit.  However, compatibility is encouraged so that VFSes can
+** interoperate.  The standard implemention used on both unix and windows
+** is for the index number to indicate a byte offset into the
+** WalCkptInfo.aLock[] array in the wal-index header.  In other words, all
+** locks are on the shm file.  The WALINDEX_LOCK_OFFSET constant (which
+** should be 120) is the location in the shm file for the first locking
+** byte.
 */
 #define WAL_WRITE_LOCK         0
 #define WAL_ALL_BUT_WRITE      1
@@ -397,7 +410,6 @@ struct WalCkptInfo {
 #define WAL_FRAME_HDRSIZE 24
 
 /* Size of write ahead log header, including checksum. */
-/* #define WAL_HDRSIZE 24 */
 #define WAL_HDRSIZE 32
 
 /* WAL magic value. Either this value, or the same value with the least