[gparted] Avoid reading the same sector multiple times in a row (!46)(#16)

[Curtis Gedak <gedakc src gnome org>]

commit 869ebb71ea06ac02422fcbecf1dc31eb50dbcb3b
Author: Mike Fleetwood <mike fleetwood googlemail com>
Date:   Fri Jul 12 10:09:50 2019 +0100

    Avoid reading the same sector multiple times in a row (!46)(#16)
    
    GParted internal file system detection is reading the same sector
    multiple times in a row.  Add an optimisation to only read a sector if
    it is different to the previously read sector.
    
    Closes !46 - Whole device FAT32 file system reports device busy warning
                 from mlabel
    Closes #16 - "invalid argument for seek()" error on very small (<=40KiB)
                 drives

 src/GParted_Core.cc | 39 +++++++++++++++++++++++++--------------
 1 file changed, 25 insertions(+), 14 deletions(-)
---
diff --git a/src/GParted_Core.cc b/src/GParted_Core.cc
index 93b17bce..7b0ecd3b 100644
--- a/src/GParted_Core.cc
+++ b/src/GParted_Core.cc
@@ -1179,6 +1179,9 @@ FSType GParted_Core::detect_filesystem_internal(const Glib::ustring& path, Byte_
        // *   Apple File System Reference
        //     https://developer.apple.com/support/apple-file-system/Apple-File-System-Reference.pdf
 
+       Byte_Value prev_read_offset = -1;
+       memset(buf, 0, sector_size);
+
        for ( unsigned int i = 0 ; i < sizeof( signatures ) / sizeof( signatures[0] ) ; i ++ )
        {
                const size_t len1 = std::min( ( signatures[i].sig1 == NULL ) ? 0U : strlen( 
signatures[i].sig1 ),
@@ -1193,26 +1196,34 @@ FSType GParted_Core::detect_filesystem_internal(const Glib::ustring& path, Byte_
 
                Byte_Value read_offset = signatures[i].offset1 / sector_size * sector_size;
 
-               memset(buf, 0, sector_size);
-               if (lseek(fd, read_offset, SEEK_SET) == read_offset &&
-                   read(fd, buf, sector_size)       == sector_size   )
+               // Optimisation: only read new sector when it is different to the
+               // previously read sector.
+               if (read_offset != prev_read_offset)
                {
-                       memcpy(magic1, buf + signatures[i].offset1 % sector_size, len1);
-
-                       // WARNING: This assumes offset2 is in the same sector as offset1
-                       if ( signatures[i].sig2 != NULL )
+                       if (lseek(fd, read_offset, SEEK_SET) == read_offset &&
+                           read(fd, buf, sector_size)       == sector_size   )
                        {
-                               memcpy(magic2, buf + signatures[i].offset2 % sector_size, len2);
+                               prev_read_offset = read_offset;
                        }
-
-                       if ( memcmp( magic1, signatures[i].sig1, len1 ) == 0     &&
-                            ( signatures[i].sig2 == NULL ||
-                              memcmp( magic2, signatures[i].sig2, len2 ) == 0 )     )
+                       else
                        {
-                               fstype = signatures[i].fstype;
-                               break;
+                               // Outside block device boundaries or other error.
+                               continue;
                        }
                }
+
+               memcpy(magic1, buf + signatures[i].offset1 % sector_size, len1);
+
+               // WARNING: This assumes offset2 is in the same sector as offset1
+               if (signatures[i].sig2 != NULL)
+                       memcpy(magic2, buf + signatures[i].offset2 % sector_size, len2);
+
+               if (memcmp(magic1, signatures[i].sig1, len1) == 0                                 &&
+                   (signatures[i].sig2 == NULL || memcmp(magic2, signatures[i].sig2, len2) == 0)   )
+               {
+                       fstype = signatures[i].fstype;
+                       break;
+               }
        }
 
        close(fd);