Calling BaseRequest.host can do blocking I/O if the host header is missing

[bdraco]

aiohttp/aiohttp/web_request.py

Line 428 in 0b8be7f

return socket.getfqdn()

There is a fallback to socket.getfqdn() which is not async safe and will block the loop while its resolved

[bdraco]

Fix for Home Assistant is home-assistant/core#126882

[bdraco]

I'm not sure how likely it would be that its not cached locally.. but if its missing from /etc/hosts it could have to resolve it

[bdraco]

Actually it appears quite likely since the first system I tested it on did blocking I/O

strace python -c "import socket; socket.getfqdn()'

open("/etc/hosts", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
read(3, "127.0.0.1\tlocalhost\n::1\tlocalhos"..., 1024) = 194
read(3, "", 1024)                       = 0
close(3)                                = 0
open("/etc/resolv.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
read(3, "# Generated by Docker Engine.\n# "..., 248) = 248
read(3, " [nameservers search options]\n", 248) = 30
read(3, "", 248)                        = 0
close(3)                                = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
bind(3, {sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
sendto(3, "\10\23\1\0\0\1\0\0\0\0\0\0\rhomeassistant\5local"..., 45, MSG_NOSIGNAL, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.30.32.3")}, 16) = 45
sendto(3, "\t\305\1\0\0\1\0\0\0\0\0\0\rhomeassistant\5local"..., 45, MSG_NOSIGNAL, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.30.32.3")}, 16) = 45
poll([{fd=-1}, {fd=-1}, {fd=3, events=POLLIN}], 3, 5000) = 1 ([{fd=3, revents=POLLIN}])
recvmsg(3, {msg_name={sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.30.32.3")}, msg_namelen=16, msg_iov=[{iov_base="\10\23\205\0\0\1\0\1\0\0\0\0\rhomeassistant\5local"..., iov_len=4800}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 88
recvmsg(3, {msg_name={sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.30.32.3")}, msg_namelen=16, msg_iov=[{iov_base="\t\305\205\0\0\1\0\0\0\0\0\0\rhomeassistant\5local"..., iov_len=4800}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 45
close(3)                                = 0
open("/etc/hosts", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
read(3, "127.0.0.1\tlocalhost\n::1\tlocalhos"..., 1024) = 194
read(3, "", 1024)                       = 0
close(3)                                = 0
open("/etc/resolv.conf", O_RDONLY|O_LARGEFILE|O_CLOEXEC) = 3
fcntl(3, F_SETFD, FD_CLOEXEC)           = 0
read(3, "# Generated by Docker Engine.\n# "..., 248) = 248
read(3, " [nameservers search options]\n", 248) = 30
read(3, "", 248)                        = 0
close(3)                                = 0
socket(AF_INET, SOCK_DGRAM|SOCK_CLOEXEC|SOCK_NONBLOCK, IPPROTO_IP) = 3
bind(3, {sa_family=AF_INET, sin_port=htons(0), sin_addr=inet_addr("0.0.0.0")}, 16) = 0
sendto(3, "O\337\1\0\0\1\0\0\0\0\0\0\0011\00232\00230\003172\7in-addr"..., 42, MSG_NOSIGNAL, {sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.30.32.3")}, 16) = 42
poll([{fd=-1}, {fd=3, events=POLLIN}], 2, 5000) = 1 ([{fd=3, revents=POLLIN}])
recvmsg(3, {msg_name={sa_family=AF_INET, sin_port=htons(53), sin_addr=inet_addr("172.30.32.3")}, msg_namelen=16, msg_iov=[{iov_base="O\337\205\0\0\1\0\n\0\0\0\0\0011\00232\00230\003172\7in-addr"..., iov_len=512}], msg_iovlen=1, msg_controllen=0, msg_flags=0}, 0) = 330
close(3)                                = 0
rt_sigaction(SIGINT, {sa_handler=SIG_DFL, sa_mask=[], sa_flags=SA_RESTORER|SA_ONSTACK, sa_restorer=0x7f324d536ea8}, {sa_handler=0x7f324cfe3de8, sa_mask=[], sa_flags=SA_RESTORER|SA_ONSTACK, sa_restorer=0x7f324d536ea8}, 8) = 0

[bdraco]

I haven't come up with a solution to this that isn't a breaking change to remove the blocking method