[gdal-dev] Logging network requests/replies?

[Even Rouault]

> Actually, what I had in mind (thinking out loud) would sit solely
> within GDAL, e.g. a method to globally specify a callback function to
> be called whenever GDAL creates a network request, e.g. within
> CPLHTTPFetchEx (and similarly for replies).

CPLHTTPFetchEx() is just one of the site that does network access. 
cpl_vsil_curl.cpp, cpl_vsil_s3.cpp, cpl_vsil_az.cpp, cpl_vsil_swift.cpp, 
cpl_vsil_webhdfs.cpp also do network access

But at the end of all them must go through CPLHTTPSetOptions() in cpl_http.cpp

> (With clients being
> responsible for ensuring that their callback functions are
> appropriately thread safe).
> 
> Does this sound desirable? Any concerns/blockers before I start hacking on
> it?

I had several thoughts about this

1) yes a dedicated callback mechanism could work of course

2) As Sean mentionned, and provided we redirect curl logging to our own 
callbacks instead of stderr, we could potentially just use the CPLDebug() 
infrastructure with the the callback being installed with CPLSetErrorHandler() 
+ CPLSetCurrentErrorHandlerCatchDebug(true). We probably wouldn't want so much 
debugging information to be emitted by default (both because of verbosity and 
performance), so this should still be conditionalized by the 
CPL_CURL_VERBOSE=YES config option/env. variable

3) An alternative would be to have a dedicated generic logging API

void CPLLogEvent(const char* topic, const char* fmt, ...);

int CPLHasLoggerFor(const char* topic);

typedef void (*CPLLoggerCallback)(const char* topic,
										 const char* msg, void* user_data);

void* CPLInstallLogger(const char* topic,
							CPLLoggerCallback cbk, void* user_data);
void CPLUninstallLogger(void*);

but I'm wondering if there wouldn't be too much confusion/intersection with 
the CPLDebug() mechanism.


My current slight preference would go to 2) to reuse existing infrastructure. 
Maybe others, including Robert CC'ed who has hacked in the error/debug area 
recently, have thoughts on this.


Even

-- 
Spatialys - Geospatial professional services
http://www.spatialys.com