[N8266-18] espconn_gethostbyname returns ERR_MEM although there is more than enough free memory

[kriegste]

How do I debug espconn_gethostbyname? I call it to retrieve the IP of "api.openweathermap.org". Sometimes (for a few hours in a row) it returns ERR_MEM although there should be more than enough memory available. I print the amount of free heap in the log and it is always around 49264 bytes more or less.

Has anyone experienced a similar behavior?

I suspect there is a fixed buffer inside the espconn_gethostbyname function which is too small for some requests. Maybe Espressif can enlighten us.

Edit:
dns is part of lwip, so I enabled debugging there and will report back new findings.

[kriegste]

The bug is here:
https://github.com/espressif/ESP8266_NONOS_SDK/blob/master/third_party/lwip/core/dns.c#L886

lseqi must not be initialized with 0, but with DNS_TABLE_SIZE. Compare the original code from the newest lwIP:
https://github.com/lwip-tcpip/lwip/blob/master/src/core/dns.c#L1440

The problem seems to be gone, but I need to monitor this for a few more days.

[kriegste]

Works.

[someburner]

@kriegste - shouldn't you leave this open until it makes it into the SDK?

[kriegste]

No problem. Don't let me stop you (or Espressif).

Looking at the new lwIP there are more bugs that need to be fixed.

[gilpinheiro]

It appears the SDK is compiled with DNS_TABLE_SIZE 0, which makes the issue moot. I base this on the latest SDK liblwip.a not containing a dns_table at all.

It'd be nice if the lwipopt.h actually reflected what options the SDK is using.

[kriegste]

The problem did not reoccur. All fine after the fix.

DNS_TABLE_SIZE is mentioned only a few times, but it is defined as 4 in the source.
https://github.com/espressif/ESP8266_NONOS_SDK/blob/master/third_party/include/lwip/opt.h#L799
https://github.com/espressif/ESP8266_NONOS_SDK/blob/master/third_party/include/lwipopts.h#L808

[FayeY]

Is this issue solved? Could we close this issue now?

[kriegste]

No, your code still has the bug.
https://github.com/espressif/ESP8266_NONOS_SDK/blob/master/third_party/lwip/core/dns.c#L886

[xcguang]

15993bf & 89693b4 & 74358b7 fix it, please try it again.