blocked on reading when output is piped (line buffering?)

When I pipe `watchman -pj --no-pretty`, it hangs reading from the socket

```
$ strace -- watchman -pj --server-encoding=json --no-pretty | cat
...
read(0, "[\"subscribe\", \"/tmp/foo\", \"foxy\""..., 1024) = 79
read(0, "", 1024) = 0
socket(PF_FILE, SOCK_STREAM, 0) = 3
connect(3, {sa_family=AF_FILE, path="/var/facebook/watchman/fugalh"}, 110) = 0
setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa4433b9000
write(3, "[\"subscribe\",\"/tmp/foo\",\"foxy\",{"..., 72) = 72
read(3, "{\"version\":\"2.9.1\",\"subscribe\":\""..., 1048576) = 702
fstat(1, {st_mode=S_IFIFO|0600, st_size=0, ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7fa4434c9000
read(3,
```

At that point it just hangs. Note the piping into cat; when it runs with a tty as stdout it works fine:

```
$ strace -- watchman -pj --server-encoding=json --no-pretty
...
read(0, "[\"subscribe\", \"/tmp/foo\", \"foxy\""..., 1024) = 79
read(0, "", 1024) = 0
socket(PF_FILE, SOCK_STREAM, 0) = 3
connect(3, {sa_family=AF_FILE, path="/var/facebook/watchman/fugalh"}, 110) = 0
setsockopt(3, SOL_SOCKET, SO_RCVBUF, [1048576], 4) = 0
mmap(NULL, 1052672, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f026bcd2000
write(3, "[\"subscribe\",\"/tmp/foo\",\"foxy\",{"..., 72) = 72
read(3, "{\"version\":\"2.9.1\",\"subscribe\":\""..., 1048576) = 702
fstat(1, {st_mode=S_IFCHR|0620, st_rdev=makedev(136, 2), ...}) = 0
mmap(NULL, 4096, PROT_READ|PROT_WRITE, MAP_PRIVATE|MAP_ANONYMOUS, -1, 0) = 0x7f026bde2000
write(1, "{\"version\":\"2.9.1\",\"subscribe\":\""..., 72) = 72
write(1, "{\"version\":\"2.9.1\",\"clock\":\"c:13"..., 630) = 630
read(3,
```

Smells like line buffering, but I'm not sure how that would make a difference here - we read 702 bytes from fd 3 both times, but the first time we try to read again instead of trying to write anything.

(Linux)