Created attachment 10628 [details]
Fix.

GDB-7.9 and GCC-5.0 contain instant code compilation feature:
  https://sourceware.org/gdb/wiki/GCCCompileAndExecute

GDB calls gcc in a way like:
  x86_64-redhat-linux-gcc -m64 -mcmodel=large -std=gnu11 -fno-exceptions -O0 -gdwarf-4 -fPIC -w -fno-stack-protector -fplugin=libcc1plugin -fplugin-arg-libcc1plugin-fd=8 /tmp/gdbobj-SBkCR2/out1.c -c -o /tmp/gdbobj-SBkCR2/out1.o

And GDB then communicates with child GCC over fd 8 (in this case).

With ccache the command hangs.  IMO it hans because it runs the command twice and so after all the communication with GDB completes second GCC tries to communicate again - which cannot work.

------------------------------------------------------------------------------
Command line: x86_64-redhat-linux-gcc -m64 -mcmodel=large -std=gnu11 -fno-exceptions -O0 -gdwarf-4 -fPIC -w -fno-stack-protector -fplugin=libcc1plugin -fplugin-arg-libcc1plugin-fd=8 /tmp/gdbobj-mhxyIe/out1.c -c -o /tmp/gdbobj-mhxyIe/out1.o
Hostname: host1.jankratochvil.net
Working directory: /home/jkratoch/redhat/gdb-clean/gdb/testsuite
Source file: /tmp/gdbobj-mhxyIe/out1.c
Object file: /tmp/gdbobj-mhxyIe/out1.o
Trying direct lookup
Looking for object file hash in /home/jkratoch/.ccache/8/f/ffd320700a68c954a27813482a8236-36287.manifest
No such manifest file
Did not find object file hash in manifest
Running preprocessor
Executing /usr/bin/x86_64-redhat-linux-gcc -m64 -mcmodel=large -std=gnu11 -fno-exceptions -O0 -gdwarf-4 -fPIC -w -fno-stack-protector -fplugin=libcc1plugin -fplugin-arg-libcc1plugin-fd=8 -E /tmp/gdbobj-mhxyIe/out1.c
Failed to stat include file gdb command line: No such file or directory
Disabling direct mode
Got object file hash from preprocessor
Object file /home/jkratoch/.ccache/d/6/a33e6857193e3619bdf58567cd261a-29788.o not in cache
Running real compiler
Executing /usr/bin/x86_64-redhat-linux-gcc -m64 -mcmodel=large -std=gnu11 -fno-exceptions -O0 -gdwarf-4 -fPIC -w -fno-stack-protector -fplugin=libcc1plugin -fplugin-arg-libcc1plugin-fd=8 -c -o /tmp/gdbobj-mhxyIe/out1.o /home/jkratoch/.ccache/tmp/out1.stdout.host1.jankratochvil.net.9782.nBRkk6.i
<hang>
------------------------------------------------------------------------------

Attaching a fix, thanks.