ptyfwd: avoid touching forwarder after exit drain

ptyfwd: avoid touching forwarder after exit drain

on_exit_event() can synchronously drain buffered data through
shovel_force(). If that completes the drain, pty_forward_done() runs
the hangup handler and may free the forwarder, so do not call
pty_forward_done() again afterwards.

[   25.052879] TEST-74-AUX-UTILS.sh[909]: ==909==ERROR: AddressSanitizer: heap-use-after-free on address 0x7ccc8a5e0b41 at pc 0x7efc8cde106e bp 0x7ffd668629b0 sp 0x7ffd668629a8
[   25.053136] TEST-74-AUX-UTILS.sh[909]: READ of size 1 at 0x7ccc8a5e0b41 thread T0
[   25.092784] TEST-74-AUX-UTILS.sh[909]:     #0 0x7efc8cde106d in pty_forward_done ../src/src/shared/ptyfwd.c:187
[   25.093920] TEST-74-AUX-UTILS.sh[909]:     #1 0x7efc8cdedba1 in on_exit_event ../src/src/shared/ptyfwd.c:904
[   25.094148] TEST-74-AUX-UTILS.sh[909]:     #2 0x7efc8d375eff in source_dispatch ../src/src/libsystemd/sd-event/sd-event.c:4301
[   25.095074] TEST-74-AUX-UTILS.sh[909]:     #3 0x7efc8d378032 in dispatch_exit ../src/src/libsystemd/sd-event/sd-event.c:4431
[   25.095295] TEST-74-AUX-UTILS.sh[909]:     #4 0x7efc8d37e932 in sd_event_dispatch ../src/src/libsystemd/sd-event/sd-event.c:4896
[   25.095467] TEST-74-AUX-UTILS.sh[909]:     #5 0x7efc8d37fc8c in sd_event_run ../src/src/libsystemd/sd-event/sd-event.c:4971
[   25.095647] TEST-74-AUX-UTILS.sh[909]:     #6 0x7efc8d3800ad in sd_event_loop ../src/src/libsystemd/sd-event/sd-event.c:4992
[   25.097174] TEST-74-AUX-UTILS.sh[909]:     #7 0x56049b541aba in start_transient_service ../src/src/run/run.c:2479
[   25.097403] TEST-74-AUX-UTILS.sh[909]:     #8 0x56049b552a65 in run ../src/src/run/run.c:3288
[   25.097569] TEST-74-AUX-UTILS.sh[909]:     #9 0x56049b552cb0 in main ../src/src/run/run.c:3291
[   25.097780] TEST-74-AUX-UTILS.sh[909]:     #10 0x7efc8b882300 in __libc_start_call_main (/lib64/libc.so.6+0x7d300) (BuildId: 830c94f480c13d9b01dc65a1035310882136094a)
[   25.097952] TEST-74-AUX-UTILS.sh[909]:     #11 0x7efc8b882417 in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x7d417) (BuildId: 830c94f480c13d9b01dc65a1035310882136094a)
[   25.098139] TEST-74-AUX-UTILS.sh[909]:     #12 0x56049b51cf54 in _start (/usr/bin/systemd-run+0x19f54) (BuildId: 0daacdb9f20151f3517312ee99e489a9b8f4989c)
[   25.098316] TEST-74-AUX-UTILS.sh[909]: 0x7ccc8a5e0b41 is located 193 bytes inside of 2384-byte region [0x7ccc8a5e0a80,0x7ccc8a5e13d0)
[   25.099202] TEST-74-AUX-UTILS.sh[909]: freed by thread T0 here:
[   25.099410] TEST-74-AUX-UTILS.sh[909]:     #0 0x7efc8e76420f in free.part.0 (/lib64/libasan.so.8+0x16420f) (BuildId: 173395e60f171589489dde2b7a156d0ae380734b)
[   25.099557] TEST-74-AUX-UTILS.sh[909]:     #1 0x7efc8cdf14d1 in pty_forward_free ../src/src/shared/ptyfwd.c:1122
[   25.099691] TEST-74-AUX-UTILS.sh[909]:     #2 0x56049b535328 in pty_forward_handler ../src/src/run/run.c:1952
[   25.100063] TEST-74-AUX-UTILS.sh[909]:     #3 0x7efc8cde138c in pty_forward_done ../src/src/shared/ptyfwd.c:196
[   25.100197] TEST-74-AUX-UTILS.sh[909]:     #4 0x7efc8cdec757 in shovel ../src/src/shared/ptyfwd.c:813
[   25.101144] TEST-74-AUX-UTILS.sh[909]:     #5 0x7efc8cdecc1f in shovel_force ../src/src/shared/ptyfwd.c:828
[   25.102273] TEST-74-AUX-UTILS.sh[909]:     #6 0x7efc8cdedb82 in on_exit_event ../src/src/shared/ptyfwd.c:899
[   25.103564] TEST-74-AUX-UTILS.sh[909]:     #7 0x7efc8d375eff in source_dispatch ../src/src/libsystemd/sd-event/sd-event.c:4301
[   25.103712] TEST-74-AUX-UTILS.sh[909]:     #8 0x7efc8d378032 in dispatch_exit ../src/src/libsystemd/sd-event/sd-event.c:4431
[   25.104081] TEST-74-AUX-UTILS.sh[909]:     #9 0x7efc8d37e932 in sd_event_dispatch ../src/src/libsystemd/sd-event/sd-event.c:4896
[   25.104954] TEST-74-AUX-UTILS.sh[909]:     #10 0x7efc8d37fc8c in sd_event_run ../src/src/libsystemd/sd-event/sd-event.c:4971
[   25.105160] TEST-74-AUX-UTILS.sh[909]:     #11 0x7efc8d3800ad in sd_event_loop ../src/src/libsystemd/sd-event/sd-event.c:4992
[   25.105310] TEST-74-AUX-UTILS.sh[909]:     #12 0x56049b541aba in start_transient_service ../src/src/run/run.c:2479
[   25.105454] TEST-74-AUX-UTILS.sh[909]:     #13 0x56049b552a65 in run ../src/src/run/run.c:3288
[   25.105572] TEST-74-AUX-UTILS.sh[909]:     #14 0x56049b552cb0 in main ../src/src/run/run.c:3291
[   25.106136] TEST-74-AUX-UTILS.sh[909]:     #15 0x7efc8b882300 in __libc_start_call_main (/lib64/libc.so.6+0x7d300) (BuildId: 830c94f480c13d9b01dc65a1035310882136094a)
[   25.106263] TEST-74-AUX-UTILS.sh[909]:     #16 0x7efc8b882417 in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x7d417) (BuildId: 830c94f480c13d9b01dc65a1035310882136094a)
[   25.106385] TEST-74-AUX-UTILS.sh[909]:     #17 0x56049b51cf54 in _start (/usr/bin/systemd-run+0x19f54) (BuildId: 0daacdb9f20151f3517312ee99e489a9b8f4989c)
[   25.106792] TEST-74-AUX-UTILS.sh[909]: previously allocated by thread T0 here:
[   25.106957] TEST-74-AUX-UTILS.sh[909]:     #0 0x7efc8e76515f in malloc (/lib64/libasan.so.8+0x16515f) (BuildId: 173395e60f171589489dde2b7a156d0ae380734b)
[   25.108013] TEST-74-AUX-UTILS.sh[909]:     #1 0x7efc8cddebed in malloc_multiply ../src/src/basic/alloc-util.h:92
[   25.108188] TEST-74-AUX-UTILS.sh[909]:     #2 0x7efc8cdee47b in pty_forward_new ../src/src/shared/ptyfwd.c:955
[   25.108324] TEST-74-AUX-UTILS.sh[909]:     #3 0x56049b538700 in run_context_setup_ptyfwd ../src/src/run/run.c:2130
[   25.108472] TEST-74-AUX-UTILS.sh[909]:     #4 0x56049b5419f9 in start_transient_service ../src/src/run/run.c:2465
[   25.109152] TEST-74-AUX-UTILS.sh[909]:     #5 0x56049b552a65 in run ../src/src/run/run.c:3288
[   25.109311] TEST-74-AUX-UTILS.sh[909]:     #6 0x56049b552cb0 in main ../src/src/run/run.c:3291
[   25.109450] TEST-74-AUX-UTILS.sh[909]:     #7 0x7efc8b882300 in __libc_start_call_main (/lib64/libc.so.6+0x7d300) (BuildId: 830c94f480c13d9b01dc65a1035310882136094a)
[   25.109847] TEST-74-AUX-UTILS.sh[909]:     #8 0x7efc8b882417 in __libc_start_main@GLIBC_2.2.5 (/lib64/libc.so.6+0x7d417) (BuildId: 830c94f480c13d9b01dc65a1035310882136094a)
[   25.110760] TEST-74-AUX-UTILS.sh[909]:     #9 0x56049b51cf54 in _start (/usr/bin/systemd-run+0x19f54) (BuildId: 0daacdb9f20151f3517312ee99e489a9b8f4989c)
[   25.110911] TEST-74-AUX-UTILS.sh[909]: SUMMARY: AddressSanitizer: heap-use-after-free ../src/src/shared/ptyfwd.c:187 in pty_forward_done
[   25.111054] TEST-74-AUX-UTILS.sh[909]: Shadow bytes around the buggy address:
[   25.111213] TEST-74-AUX-UTILS.sh[909]:   0x7ccc8a5e0880: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
[   25.111378] TEST-74-AUX-UTILS.sh[909]:   0x7ccc8a5e0900: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
[   25.111520] TEST-74-AUX-UTILS.sh[909]:   0x7ccc8a5e0980: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
[   25.112210] TEST-74-AUX-UTILS.sh[909]:   0x7ccc8a5e0a00: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
[   25.112399] TEST-74-AUX-UTILS.sh[909]:   0x7ccc8a5e0a80: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
[   25.112767] TEST-74-AUX-UTILS.sh[909]: =>0x7ccc8a5e0b00: fd fd fd fd fd fd fd fd[fd]fd fd fd fd fd fd fd
[   25.112901] TEST-74-AUX-UTILS.sh[909]:   0x7ccc8a5e0b80: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
[   25.113789] TEST-74-AUX-UTILS.sh[909]:   0x7ccc8a5e0c00: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
[   25.113906] TEST-74-AUX-UTILS.sh[909]:   0x7ccc8a5e0c80: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
[   25.114046] TEST-74-AUX-UTILS.sh[909]:   0x7ccc8a5e0d00: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
[   25.114159] TEST-74-AUX-UTILS.sh[909]:   0x7ccc8a5e0d80: fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd fd
[   25.114278] TEST-74-AUX-UTILS.sh[909]: Shadow byte legend (one shadow byte represents 8 application bytes):
[   25.114400] TEST-74-AUX-UTILS.sh[909]:   Addressable:           00
[   25.115099] TEST-74-AUX-UTILS.sh[909]:   Partially addressable: 01 02 03 04 05 06 07
[   25.115246] TEST-74-AUX-UTILS.sh[909]:   Heap left redzone:       fa
[   25.115365] TEST-74-AUX-UTILS.sh[909]:   Freed heap region:       fd
[   25.115483] TEST-74-AUX-UTILS.sh[909]:   Stack left redzone:      f1
[   25.115618] TEST-74-AUX-UTILS.sh[909]:   Stack mid redzone:       f2
[   25.115882] TEST-74-AUX-UTILS.sh[909]:   Stack right redzone:     f3
[   25.116735] TEST-74-AUX-UTILS.sh[909]:   Stack after return:      f5
[   25.116857] TEST-74-AUX-UTILS.sh[909]:   Stack use after scope:   f8
[   25.116974] TEST-74-AUX-UTILS.sh[909]:   Global redzone:          f9
[   25.117108] TEST-74-AUX-UTILS.sh[909]:   Global init order:       f6
[   25.117257] TEST-74-AUX-UTILS.sh[909]:   Poisoned by user:        f7
[   25.118128] TEST-74-AUX-UTILS.sh[909]:   Container overflow:      fc
[   25.118288] TEST-74-AUX-UTILS.sh[909]:   Array cookie:            ac
[   25.118433] TEST-74-AUX-UTILS.sh[909]:   Intra object redzone:    bb
[   25.118546] TEST-74-AUX-UTILS.sh[909]:   ASan internal:           fe
[   25.118684] TEST-74-AUX-UTILS.sh[909]:   Left alloca redzone:     ca
[   25.118792] TEST-74-AUX-UTILS.sh[909]:   Right alloca redzone:    cb
[   25.119282] TEST-74-AUX-UTILS.sh[909]: Command: systemd-run --quiet --pty -- bash -c echo PTY_FORWARD_READY; exec sleep 60
[   25.119395] TEST-74-AUX-UTILS.sh[909]: ==909==ABORTING

Follow-up for d3218c4c80c99583d3e7a31ff2f509ffb097568e