Go: too many open files

Recently while creating a basic HTTP/HTTPS monitoring app, Pingo2, I started seeing too many open files error. This error was thrown after the app had been running for some time, and I attempted to open a new network connection.

Of course, in Unix/Linux network sockets are just files, so this error message actually makes sense in that context. First thing to do was run lsof to see exactly which files the process had open:

lsof | grep 19991
myapp    19991 20009 monitoring   38u     IPv4            4685252       0t0        TCP dev.example.com:44449->foobar:https (ESTABLISHED)
myapp    19991 20009 monitoring   39u     IPv4            4685250       0t0        TCP dev.example.com:45459->xxx.xxx.189.184:https (ESTABLISHED)
myapp    19991 20009 monitoring   40u     IPv4            4685251       0t0        TCP dev.example.com:45460->xxx.xxx.189.184:https (ESTABLISHED)
myapp    19991 20009 monitoring   41u     IPv4            4685253       0t0        TCP dev.example.com:44450->foobar:https (ESTABLISHED)
myapp    19991 20009 monitoring   42u     IPv4            4685268       0t0        TCP dev.example.com:44454->foobar:https (ESTABLISHED)
myapp    19991 20009 monitoring   43u     IPv4            4685266       0t0        TCP dev.example.com:45464->xxx.xxx.189.184:https (ESTABLISH

So, lots of ESTABLISHED network connections, each one corresponding with a HTTP connection my program had initiated. Now I was explicitly closing the HTTP response body after each connection, and thought that was sufficient for the connection to close down by itself. However it turns out that the default HTTP transport has TCP keep-alives enabled. The TCP connections were piling up in the background as a result.

Creating a custom http.Transport for the HTTP client with DisableKeepAlives: true fixed the issue.