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Subject cvs commit: apache-2.0/src/lib/apr/docs canonical_filenames.html
Date Mon, 06 Nov 2000 18:04:29 GMT
wrowe       00/11/06 10:04:26

  Added:       src/lib/apr/docs canonical_filenames.html
    I've committed this document so that others (esp. Mac OS X, NetWare, OS2, etc)
    can refine the observations about their platforms' path processing behavior.
    Why html?  Next revision from me will define the Win32/OS2 pseudo-code in
    an alternate style, to highlight differences and discrepancies.
  Revision  Changes    Path
  1.1                  apache-2.0/src/lib/apr/docs/canonical_filenames.html
  Index: canonical_filenames.html
  <h1>Canonical Filename</h1>
  <p>APR porters need to address the underlying discrepancies between
  file systems.  To achieve a reasonable degree of security, the
  program depending upon APR needs to know that two paths may be
  compared, and that a mismatch is guarenteed to reflect that the
  two paths do not return the same resource</p>.
  <p>The first discrepancy is in volume roots.  Unix and pure deriviates
  have only one root path, "/".  Win32 and OS2 share root paths of
  the form "D:/", D: is the volume designation.  However, this can
  be specified as "//./D:/" as well, indicating D: volume of the 
  'this' machine.  Win32 and OS2 also may employ a UNC root path,
  of the form "//server/share/" where share is a share-point of the
  specified network server.  Finally, NetWare root paths are of the
  form "server/volume:/", or the simpler "volume:/" syntax for 'this'
  machine.  All these non-Unix file systems accept volume:path,
  without a slash following the colon, as a path relative to the
  current working directory, which APR will treat as ambigious, that
  is, neither an absolute nor a relative path per se.</p>
  <p>The second discrepancy is in the meaning of the 'this' directory.
  In general, 'this' must be eliminated from the path where it occurs.
  The syntax "path/./" and "path/" are both aliases to path.  However,
  this isn't file system independent, since the double slash "//" has
  a special meaning on OS2 and Win32 at the start of the path name,
  and is invalid on those platforms before the "//server/share/" UNC
  root path is completed.  Finally, as noted above, "//./volume/" is
  legal root syntax on WinNT, and perhaps others.</p>
  <p>The third discrepancy is in the context of the 'parent' directory.
  When "parent/path/.." occurs, the path must be unwound to "parent".
  It's also critical to simply truncate leading "/../" paths to "/",
  since the parent of the root is root.  This gets tricky on the
  Win32 and OS2 platforms, since the ".." element is invalid before
  the "//server/share/" is complete, and the "//server/share/../"
  seqence is the complete UNC root "//server/share/".  In relative
  paths, leading ".." elements are significant, until they are merged
  with an absolute path.  The relative form must only retain the ".."
  segments as leading segments, to be resolved once merged to another
  relative or an absolute path.</p>
  <p>The fourth discrepancy occurs with acceptance of alternate character
  codes for the same element.  Path seperators are not retained within
  the APR canonical forms.  The OS filesystem and APR (slashed) forms
  can both be returned as strings, to be used in the proper context.
  Unix, Win32 and Netware all accept slashes and backslashes as the
  same path seperator symbol, although unix strictly accepts slashes.
  While the APR form of the name strictly uses slashes, always consider
  that there could be a platform that actually accepts slashes as a
  character within a segment name.</p>
  <p>The fifth and worst discrepancy plauges Win32, OS2, Netware, and some
  filesystems mounted in Unix.  Case insensitivity can permit the same
  file to slip through in both it's proper case and alternate cases.
  Simply changing the case is insufficient for any character set beyond
  ASCII, since various dilectic forms of characters suffer from one to
  many or many to one translations.  An example would be u-umlaut, which
  might be accepted as a single character u-umlaut, a two character
  sequence u and the zero-width umlaut, the upper case form of the same,
  or perhaps even a captial U alone.  This can be handled in different
  ways depending on the purposes of the APR based program, but the one
  requirement is that the path must be absolute in order to resolve these
  ambiguities.  Methods employed include comparison of device and inode
  file uniqifiers, which is a fairly fast operation, or quering the OS
  for the true form of the name, which can be much slower.  Only the
  acknowledgement of the file names by the OS can validate the equality
  of two different cases of the same filename.</p>
  <p>The sixth discrepancy, illegal or insignificant characters, is especially 
  significant in non-unix file systems.  Trailing periods are accepted
  but never stored, therefore trailing periods must be ignored for any
  form of comparison.  And all OS's have certain expectations of what
  characters are illegal (or undesireable due to confusion.)</p>
  <p>A final warning, canonical functions don't transform or resolve case
  or character ambiguity issues until they are resolved into an absolute
  path.  The relative canonical path, while useful, while useful for URL
  or similar identifiers, cannot be used for testing or comparison of file 
  system objects.</p>
  <h2>Canonical API</h2>
  Functions to manipulate the apr_canon_file_t (an opaque type) include:
  <li>Create canon_file_t (from char* path and canon_file_t parent path)
  <li>Merged canon_file_t (from path and parent, both canon_file_t)
  <li>Get char* path of all or some segments
  <li>Get path flags of IsRelative, IsVirtualRoot, and IsAbsolute
  <li>Compare two canon_file_t structures for file equality
  <p>The path is corrected to the file system case only if is in absolute 
  form.  The apr_canon_file_t should be preserved as long as possible and 
  used as the parent to create child entries to reduce the number of expensive 
  stat and case canonicalization calls to the OS.</p>
  <p>The comparison operation provides that the APR can postpone correction
  of case by simply relying upon the device and inode for equivilance.  The
  stat implementation provides that two files are the same, while their
  strings are not equivilant, and eliminates the need for the operating
  system to return the proper form of the name.</p>
  <p>In any case, returning the char* path, with a flag to request the proper
  case, forces the OS calls to resolve the true names of each segment.  Where
  there is a penality for this operation and the stat device and inode test
  is faster, case correction is postponed until the char* result is requested.
  On platforms that identify the inode, device, or proper name interchangably
  with no penalities, this may occur when the name is initially processed.</p>
  <h2>Unix Example</h2>
  <p>First the simplest case:</p>
  Parse Canonical Name 
  accepts parent path as canonical_t
          this path as string
  Split this path Segments on '/'
  For each of this path Segments
    If first Segment
      If this Segment is Empty ([nothing]/)
        Append this Root Segment (don't merge)
        Continue to next Segment
      Else is relative
        Append parent Segments (to merge)
        Continue with this Segment
    If Segment is '.' or empty (2 slashes)
      Discard this Segment
      Continue with next Segment
    If Segment is '..'
      If no previous Segment or previous Segment is '..'
        Append this Segment
        Continue with next Segment
      If previous Segment and previous is not Root Segment
        Discard previous Segment
      Discard this Segment
      Continue with next Segment
    Append this Relative Segment
    Continue with next Segment        

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