Skip file or archive member if its transformed name is empty.

* NEWS: Document changes.
* doc/tar.texi: Document changes.
* src/common.h (transform_stat_info): Change return value.
(transform_name_fp): Change signature.
(WARN_EMPTY_TRANSFORM): New constant.
* src/create.c: Check return from transform_name.  Skip file, if it
is false.
* src/list.c (transform_stat_info): Return bool.
(read_and): Skip member if transform_stat_info returns false.
* src/transform.c (_transform_name_to_obstack): Change return type.
Always allocate result in obstack.
(transform_name_fp): Change arguments.  Return true on
success (transformed string not empty).  Otherwise return false and
don't change the source string.
* src/warning.c: New warning class: empty-transform.
* tests/extrac17.at: Use --warning=empty-transform.

doc/tar.texi

@@ -4639,7 +4639,7 @@ The subsections below discuss allowed values for @var{keyword} along with the
 warning messages they control.
 
 @menu
-* General Warnings::  Keywords applicable for @command{tar --create}.
+* General Warnings::  Keywords controlling @command{tar} operation.
 * Archive Creation Warnings:: Keywords applicable for @command{tar --create}.
 * Archive Extraction Warnings:: Keywords applicable for @command{tar --extract}.
 * Incremental Extraction Warnings:: Keywords controlling incremental extraction.
@@ -4670,6 +4670,11 @@ suppressed if @option{--ignore-zeros} is in effect (@pxref{Ignore
 Zeros}).
 @end defvr
 
+@defvr {warning} empty-transform
+@cindex @samp{transforms to empty name}, warning message.
+@samp{transforms to empty name}.  @xref{transform}.
+@end defvr
+
 @defvr {warning} missing-zero-blocks
 @cindex @samp{Terminating zero blocks missing}, warning message.
 @samp{Terminating zero blocks missing at %s}.  This warning is
@@ -8906,7 +8911,9 @@ $ @kbd{tar -xf usr.tar --strip=2 usr/include/stdlib.h}
 
 The option @option{--strip=2} instructs @command{tar} to strip the
 two leading components (@file{usr/} and @file{include/}) off the file
-name.
+name, before extracting it.  Notice, that archive members to extract are
+searched before that modification, hence the file name is specified in
+full.
 
 If you add the @option{--verbose} (@option{-v}) option to the invocation
 above, you will note that the verbose listing still contains the
@@ -9022,7 +9029,6 @@ follows the GNU @command{sed} implementation in this regard, so
 the interaction is defined to be: ignore matches before the
 @var{number}th, and then match and replace all matches from the
 @var{number}th on.
-
 @end table
 
 In addition, several @dfn{transformation scope} flags are supported,
@@ -9162,6 +9168,42 @@ $ @kbd{tar -cf arch.tar \
                --transform='s,/usr/var,/var/;s,/usr/local,/usr/,'}
 @end smallexample
 
+Applying transformations to some file names may produce empty
+strings.  This may indicate errors in regular expressions, but it may
+as well happen during normal operation, for instance, when using
+@option{--strip-components} option with a value which is greater then
+or equal to the number of directory components in the topmost
+directory stored in the archive.  Consider the following example.  Let
+the archive @file{usr.tar} contain:
+
+@example
+@group
+$ @kbd{tar tf usr.tar}
+usr/
+usr/local/
+...
+@end group
+@end example
+
+Extracting from this archive with the @option{--strip=1} option will
+transform the name of its first entry (@samp{usr/}) to an empty
+string.  When this happens, @GNUTAR{} prints a warning and skips this
+member:
+
+@example
+@group
+$ @kbd{tar --strip=1 -xf usr.tar}
+tar: usr: transforms to empty name
+@end group
+@end example
+
+If an empty name results from transformations applied when creating or
+updating the archive, the warning is output and the file is not
+archived.
+
+If this is intended, you can suppress the warning using the
+@option{--warning=no-empty-transform} option (@pxref{warnings}).
+
 @node after
 @section Operating Only on New Files