# Confirmed that the workaround works in practice.

I had a hang, figured out which /usr/bin/script process was eating the CPU, then issued:

head -n 1 /proc/7514/fd/4 

and it continued working.

# Try to figure out what happens.

Below is after the problem was fixed (things worked well again, the terminal running /usr/bin/script has some text flowing at all times).

In another terminal I typed the commands below

{ while sleep 1 ; do date ; done } & # First, get some timestamps to see where program pauses
strace head -n 1 /proc/7514/fd/4 

And observed the output below (shortened):

open("/proc/7514/fd/4", O_RDONLY)       = 3
read(3, jeudi 7 avril 2016, 14:30:02 (UTC+0200)
jeudi 7 avril 2016, 14:30:03 (UTC+0200)
jeudi 7 avril 2016, 14:30:04 (UTC+0200)

"head" sat waiting until I typed something in the terminal running /usr/bin/script.

jeudi 7 avril 2016, 14:30:05 (UTC+0200)
jeudi 7 avril 2016, 14:30:06 (UTC+0200)
"\n", 8192)                     = 1
lseek(3, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
fstat(3, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 12), ...}) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 4), ...}) = 0
write(1, "\n", 1
)                       = 1
close(3)                                = 0
close(1)                                = 0
close(2)                                = 0
exit_group(0)                           = ?



I'll compare with what happens next time /usr/bin/script hangs.




Full output

execve("/usr/bin/head", ["head", "-n", "1", "/proc/7514/fd/4"], [/* 72 vars */]) = 0
brk(NULL)                               = 0xde4000
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
mmap(NULL, 8192, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f35d956c000
access("/etc/ld.so.preload", R_OK)      = -1 ENOENT (No such file or directory)
open("/etc/ld.so.cache", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=123250, ...}) = 0
mmap(NULL, 123250, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f35d954d000
close(3)                                = 0
access("/etc/ld.so.nohwcap", F_OK)      = -1 ENOENT (No such file or directory)
open("/lib/x86_64-linux-gnu/libc.so.6", O_RDONLY|O_CLOEXEC) = 3
read(3, "\177ELF\2\1\1\3\0\0\0\0\0\0\0\0\3\0>\0\1\0\0\0`\t\2\0\0\0\0\0"..., 832) = 832
fstat(3, {st_mode=S_IFREG|0755, st_size=1868984, ...}) = 0
mmap(NULL, 3971584, PROT_READ|PROT_EXEC, MAP_PRIVATE|MAP_DENYWRITE, 3, 0) = 0x7f35d8f7f000
mprotect(0x7f35d913f000, 2097152, PROT_NONE) = 0
mmap(0x7f35d933f000, 24576, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_DENYWRITE, 3, 0x1c0000) = 0x7f35d933f000
mmap(0x7f35d9345000, 14848, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_FIXED|MAP_ANONYMOUS, -1, 0) = 0x7f35d9345000
close(3)                                = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f35d954c000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f35d954b000
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f35d954a000
arch_prctl(ARCH_SET_FS, 0x7f35d954b700) = 0
mprotect(0x7f35d933f000, 16384, PROT_READ) = 0
mprotect(0x608000, 4096, PROT_READ)     = 0
mprotect(0x7f35d956e000, 4096, PROT_READ) = 0
munmap(0x7f35d954d000, 123250)          = 0
brk(NULL)                               = 0xde4000
brk(0xe05000)                           = 0xe05000
open("/usr/lib/locale/locale-archive", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=3001856, ...}) = 0
mmap(NULL, 3001856, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f35d8ca2000
close(3)                                = 0
open("/usr/share/locale/locale.alias", O_RDONLY|O_CLOEXEC) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=2995, ...}) = 0
read(3, "# Locale name alias data base.\n#"..., 4096) = 2995
read(3, "", 4096)                       = 0
close(3)                                = 0
open("/usr/share/locale/fr_FR/LC_MESSAGES/coreutils.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale/fr/LC_MESSAGES/coreutils.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/fr_FR/LC_MESSAGES/coreutils.mo", O_RDONLY) = -1 ENOENT (No such file or directory)
open("/usr/share/locale-langpack/fr/LC_MESSAGES/coreutils.mo", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=297935, ...}) = 0
mmap(NULL, 297935, PROT_READ, MAP_PRIVATE, 3, 0) = 0x7f35d9501000
close(3)                                = 0
open("/usr/lib/x86_64-linux-gnu/gconv/gconv-modules.cache", O_RDONLY) = 3
fstat(3, {st_mode=S_IFREG|0644, st_size=26258, ...}) = 0
mmap(NULL, 26258, PROT_READ, MAP_SHARED, 3, 0) = 0x7f35d9565000
close(3)                                = 0
open("/proc/7514/fd/4", O_RDONLY)       = 3
read(3, jeudi 7 avril 2016, 14:30:02 (UTC+0200)
jeudi 7 avril 2016, 14:30:03 (UTC+0200)
jeudi 7 avril 2016, 14:30:04 (UTC+0200)
jeudi 7 avril 2016, 14:30:05 (UTC+0200)
jeudi 7 avril 2016, 14:30:06 (UTC+0200)
"\n", 8192)                     = 1
lseek(3, 0, SEEK_CUR)                   = -1 ESPIPE (Illegal seek)
fstat(3, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 12), ...}) = 0
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 4), ...}) = 0
write(1, "\n", 1
)                       = 1
close(3)                                = 0
close(1)                                = 0
close(2)                                = 0
exit_group(0)                           = ?
+++ exited with 0 +++