Add the SQLITE_IOCAP_SUBPAGE_READ bit to the possible returns values

from xDeviceCharacteristics method of the system-IO abstract class.  The
direct-overflow-read optimization is disabled for any VFS that does not
set this bit.  The bit is set for standard VFSes.

FossilOrigin-Name: 6e7d498cda1d8fa76f3efac0ee0540dd8241db75e85b863097cab2ed530aabb0

manifest

@@ -1,5 +1,5 @@
-C Do\snot\stypedef\sTcl_Size\sif\sit\sis\salready\s#defined.
-D 2024-10-22T16:19:14.101
+C Add\sthe\sSQLITE_IOCAP_SUBPAGE_READ\sbit\sto\sthe\spossible\sreturns\svalues\nfrom\sxDeviceCharacteristics\smethod\sof\sthe\ssystem-IO\sabstract\sclass.\s\sThe\ndirect-overflow-read\soptimization\sis\sdisabled\sfor\sany\sVFS\sthat\sdoes\snot\nset\sthis\sbit.\s\sThe\sbit\sis\sset\sfor\sstandard\sVFSes.
+D 2024-10-22T19:33:20.597
 F .fossil-settings/empty-dirs dbb81e8fc0401ac46a1491ab34a7f2c7c0452f2f06b54ebb845d024ca8283ef1
 F .fossil-settings/ignore-glob 35175cdfcf539b2318cb04a9901442804be81cd677d8b889fcc9149c21f239ea
 F LICENSE.md df5091916dbb40e6e9686186587125e1b2ff51f022cc334e886c19a0e9982724
@@ -756,10 +756,10 @@ F src/os.h 1ff5ae51d339d0e30d8a9d814f4b8f8e448169304d83a7ed9db66a65732f3e63
 F src/os_common.h 6c0eb8dd40ef3e12fe585a13e709710267a258e2c8dd1c40b1948a1d14582e06
 F src/os_kv.c 4d39e1f1c180b11162c6dc4aa8ad34053873a639bac6baae23272fc03349986a
 F src/os_setup.h 6011ad7af5db4e05155f385eb3a9b4470688de6f65d6166b8956e58a3d872107
-F src/os_unix.c 14d0f80e4779f5f76bcc71e7af97e3efe318fa3d0b22335623c59fab7d39302b
-F src/os_win.c 6ff43bac175bd9ed79e7c0f96840b139f2f51d01689a638fd05128becf94908a
+F src/os_unix.c 0ad4e0885294b3a0e135a18533590ec9ad91ffe82f6a08e55b40babd51772928
+F src/os_win.c 69fa1aaff68270423c85cff4327ba17ef99a1eb017e1a2bfb97416d9b8398b05
 F src/os_win.h 7b073010f1451abe501be30d12f6bc599824944a
-F src/pager.c b08600ebf0db90b6d1e9b8b6577c6fa3877cbe1a100bd0b2899e4c6e9adad4b3
+F src/pager.c d57287cd0864c88740d20618e5e595b144b85bfdaefccafd353d596f254a7b35
 F src/pager.h 4b1140d691860de0be1347474c51fee07d5420bd7f802d38cbab8ea4ab9f538a
 F src/parse.y a7a8d42eeff01d267444ddb476029b0b1726fb70ae3d77984140f17ad02e2d61
 F src/pcache.c 588cc3c5ccaaadde689ed35ce5c5c891a1f7b1f4d1f56f6cf0143b74d8ee6484
@@ -774,7 +774,7 @@ F src/resolve.c c8a5372b97b2a2e972a280676f06ddb5b74e885d3b1f5ce383f839907b57ef68
 F src/rowset.c 8432130e6c344b3401a8874c3cb49fefe6873fec593294de077afea2dce5ec97
 F src/select.c 4b14337a2742f0c0beeba490e9a05507e9b4b12184b9cd12773501d08d48e3fe
 F src/shell.c.in 0662f9bcf0725461778d0254a06150e5d61c08c5a87a7281ccdf45552050c79d
-F src/sqlite.h.in add9e064d6b42af8f1a4a3322bddadec76696e520aedebd83e0d3211c15ac999
+F src/sqlite.h.in 29fc900a58f394c7488d093fd7a8dcb14d3fa6399d5178cb20adcf88dbedfe39
 F src/sqlite3.rc 5121c9e10c3964d5755191c80dd1180c122fc3a8
 F src/sqlite3ext.h 3f046c04ea3595d6bfda99b781926b17e672fd6d27da2ba6d8d8fc39981dcb54
 F src/sqliteInt.h baae24292817e13e7fe748851c62efc381dcc4dac241b1182eac3d2f05eae52c
@@ -2219,8 +2219,9 @@ F vsixtest/vsixtest.tcl 6195aba1f12a5e10efc2b8c0009532167be5e301abe5b31385638080
 F vsixtest/vsixtest.vcxproj.data 2ed517e100c66dc455b492e1a33350c1b20fbcdc
 F vsixtest/vsixtest.vcxproj.filters 37e51ffedcdb064aad6ff33b6148725226cd608e
 F vsixtest/vsixtest_TemporaryKey.pfx e5b1b036facdb453873e7084e1cae9102ccc67a0
-P e2bd3219d9f7bab377ebcfa9a737ca59899c68dad1e3d1d16347bbfdd25652ee
-R ca8802466a5e5918921ffde8ccadae70
+P 53491688cf622ca317b3ff56156e601c2bdfffc94e4fe471ca82d5ba1d9e875f dd446ef1816f4e95ce29de0b3841dd0a6c983646fd2dd271c2db8f3263478164
+R ca0e4d7e96f02509781264c3f3c418db
+T +closed dd446ef1816f4e95ce29de0b3841dd0a6c983646fd2dd271c2db8f3263478164
 U drh
-Z ec07baffb11da349364a8e022541c4d6
+Z c62b27a13345423a196a9df8f7a1e878
 # Remove this line to create a well-formed Fossil manifest.

manifest.uuid

@@ -1 +1 @@
-53491688cf622ca317b3ff56156e601c2bdfffc94e4fe471ca82d5ba1d9e875f
+6e7d498cda1d8fa76f3efac0ee0540dd8241db75e85b863097cab2ed530aabb0

src/os_unix.c

@@ -4128,6 +4128,7 @@ static void setDeviceCharacteristics(unixFile *pFd){
     if( pFd->ctrlFlags & UNIXFILE_PSOW ){
       pFd->deviceCharacteristics |= SQLITE_IOCAP_POWERSAFE_OVERWRITE;
     }
+    pFd->deviceCharacteristics |= SQLITE_IOCAP_SUBPAGE_READ;
 
     pFd->sectorSize = SQLITE_DEFAULT_SECTOR_SIZE;
   }

src/os_win.c

@@ -3660,7 +3660,7 @@ static int winSectorSize(sqlite3_file *id){
 */
 static int winDeviceCharacteristics(sqlite3_file *id){
   winFile *p = (winFile*)id;
-  return SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN |
+  return SQLITE_IOCAP_UNDELETABLE_WHEN_OPEN | SQLITE_IOCAP_SUBPAGE_READ |
          ((p->ctrlFlags & WINFILE_PSOW)?SQLITE_IOCAP_POWERSAFE_OVERWRITE:0);
 }
 

src/pager.c

@@ -808,18 +808,26 @@ static const unsigned char aJournalMagic[] = {
 ** Return true if page pgno can be read directly from the database file
 ** by the b-tree layer. This is the case if:
 **
-**   * the database file is open,
-**   * there are no dirty pages in the cache, and
-**   * the desired page is not currently in the wal file.
+**   (1)  the database file is open
+**   (2)  the VFS for the database is able to do unaligned sub-page reads
+**   (3)  there are no dirty pages in the cache, and
+**   (4)  the desired page is not currently in the wal file.
 */
 int sqlite3PagerDirectReadOk(Pager *pPager, Pgno pgno){
-  if( pPager->fd->pMethods==0 ) return 0;
-  if( sqlite3PCacheIsDirty(pPager->pPCache) ) return 0;
+  assert( pPager!=0 );
+  assert( pPager->fd!=0 );
+  if( pPager->fd->pMethods==0 ) return 0;  /* Case (1) */
+  assert( pPager->fd->pMethods->xDeviceCharacteristics!=0 );
+  if( (pPager->fd->pMethods->xDeviceCharacteristics(pPager->fd)
+        & SQLITE_IOCAP_SUBPAGE_READ)==0 ){
+    return 0; /* Case (2) */
+  }
+  if( sqlite3PCacheIsDirty(pPager->pPCache) ) return 0; /* Failed (3) */
 #ifndef SQLITE_OMIT_WAL
   if( pPager->pWal ){
     u32 iRead = 0;
     (void)sqlite3WalFindFrame(pPager->pWal, pgno, &iRead);
-    return iRead==0;
+    return iRead==0; /* Condition (4) */
   }
 #endif
   return 1;

src/sqlite.h.in

@@ -652,6 +652,13 @@ int sqlite3_exec(
 ** filesystem supports doing multiple write operations atomically when those
 ** write operations are bracketed by [SQLITE_FCNTL_BEGIN_ATOMIC_WRITE] and
 ** [SQLITE_FCNTL_COMMIT_ATOMIC_WRITE].
+**
+** The SQLITE_IOCAP_SUBPAGE_READ property means that it is ok to read
+** from the database file in amounts that are not a multiple of the
+** page size and that do not begin at a page boundary.  Without this
+** property, SQLite is careful to only do full-page reads and write
+** on aligned pages, with the one exception that it will do a sub-page
+** read of the first page to access the database header.
 */
 #define SQLITE_IOCAP_ATOMIC                 0x00000001
 #define SQLITE_IOCAP_ATOMIC512              0x00000002
@@ -668,6 +675,7 @@ int sqlite3_exec(
 #define SQLITE_IOCAP_POWERSAFE_OVERWRITE    0x00001000
 #define SQLITE_IOCAP_IMMUTABLE              0x00002000
 #define SQLITE_IOCAP_BATCH_ATOMIC           0x00004000
+#define SQLITE_IOCAP_SUBPAGE_READ           0x00008000
 
 /*
 ** CAPI3REF: File Locking Levels
@@ -814,6 +822,7 @@ struct sqlite3_file {
 ** <li> [SQLITE_IOCAP_POWERSAFE_OVERWRITE]
 ** <li> [SQLITE_IOCAP_IMMUTABLE]
 ** <li> [SQLITE_IOCAP_BATCH_ATOMIC]
+** <li> [SQLITE_IOCAP_SUBPAGE_READ]
 ** </ul>
 **
 ** The SQLITE_IOCAP_ATOMIC property means that all writes of