Created attachment 478893 [details]
Reproducer

Created attachment 478903 [details]
patch-reproducer.tar.gz (but works for me)

I can't reproduce this.

patch-2.6.1-8.fc15.x86_64

Attached is the complete reproducer I tried (not just the patch: the patch, the file, and the invocation).

I obtained the file to patch by adding "exit 1" before "%patch3 -p0" in the spec file before running 'fedpkg prep'.

Backup files are written when the patch did not match the file exactly (see the --backup-if-mismatch option).

Does the reproducer fail for you?

Unfortunately I cannot reproduce it using your reproducer. While I can reproduce it always using "mock -r fedora-rawhide-x86_64 rubygem-actionpack-3.0.3-3.fc15.src.rpm". 

Not sure where is the problem :( I have tried to export the environment right after the failure if it helps:

+ test -f specifications/actionpack-3.0.3.gemspec.orig
+ echo PASS: no .orig file
PASS: no .orig file
+ echo 'Patch #3 (actionpack-downgrade-dependencies.patch):'
Patch #3 (actionpack-downgrade-dependencies.patch):
+ /bin/cat /builddir/build/SOURCES/actionpack-downgrade-dependencies.patch
+ /usr/bin/patch -s -p0 --fuzz=0
+ test -f specifications/actionpack-3.0.3.gemspec.orig
+ echo FAIL: .orig file present
FAIL: .orig file present
+ ls -lZ specifications/
-rw-r--r--. root root unconfined_u:object_r:mock_var_lib_t:s0 actionpack-3.0.3.gemspec
-rw-r--r--. root root unconfined_u:object_r:mock_var_lib_t:s0 actionpack-3.0.3.gemspec.orig
+ export
export HOME="/builddir"
export LANG="C"
export LOGNAME="root"
export OLDPWD="/builddir/build/BUILD/rubygem-actionpack-3.0.3"
export PATH="/usr/sbin:/usr/bin:/sbin:/bin:/root/bin"
export PKG_CONFIG_PATH="/usr/lib64/pkgconfig:/usr/share/pkgconfig"
export PWD="/builddir/build/BUILD/rubygem-actionpack-3.0.3/usr/lib/ruby/gems/1.8"
export RPM_ARCH="x86_64"
export RPM_BUILD_DIR="/builddir/build/BUILD"
export RPM_BUILD_ROOT="/builddir/build/BUILDROOT/rubygem-actionpack-3.0.3-3.fc16.x86_64"
export RPM_DOC_DIR="/usr/share/doc"
export RPM_OPT_FLAGS="-O2 -g -pipe -Wall -Wp,-D_FORTIFY_SOURCE=2 -fexceptions -fstack-protector --param=ssp-buffer-size=4 -m64 -mtune=generic"
export RPM_OS="linux"
export RPM_PACKAGE_NAME="rubygem-actionpack"
export RPM_PACKAGE_RELEASE="3.fc16"
export RPM_PACKAGE_VERSION="3.0.3"
export RPM_SOURCE_DIR="/builddir/build/SOURCES"
export SHELL="/bin/sh"
export SHLVL="3"
export TERM="xterm"
export USER="root"
export USERHELPER_UID="501"
+ exit 1

Created attachment 478942 [details]
script executed by rpmbuild

The attached script is the script which is executing the rpmbuild to build the package if it may help.

Created attachment 479092 [details]
Valid reproducer

I found the difference. There was introduced RubyGems 1.5 into F15/Rawhide which produced a little bit different .gemspec file. There is missing "current_version = Gem::Specification::CURRENT_SPECIFICATION_VERSION" line. So the truth is that the patch doesn't apply cleanly. 

However, there is not error message nor anything else (except the .orig file) which would point that out. Is that intentional? Shouldn't be there some warning at least? It would make life easier IMO.

rpmbuild deliberately stops patch from telling us this by passing the '-s' flag...

Taking out the '-s' from runme.sh gives this output:

$ ./runme.sh 
patching file specifications/actionpack-3.0.3.gemspec
Hunk #1 succeeded at 26 (offset -1 lines).
FAIL: .orig file present

That "(offset ... lines)" message is the clue that the patch doesn't match the file exactly.

So yes, this is all intentional, but obviously can lead to a confusing situation if the presence of a backup file causes a problem.

One solution might be for rpmbuild to tell patch not to create backup files in this situation by using the --no-backup-if-mismatch option; another might be for it to omit the -s option.

Tim, thank you for your help. I have created #678000 for rpmdevtools to fix
this issue.