[PROF-5343] Isolate profiling native extension native symbols

[ivoanjo]

When Ruby loads a native extension (using require), it uses dlopen(..., RTLD_LAZY | RTLD_GLOBAL) (https://github.com/ruby/ruby/blob/67950a4c0a884bdb78d9beb4405ebf7459229b21/dln.c#L362).

This means that every symbol exposed directly or indirectly by that native extension becomes visible to every other extension in the Ruby process. This can cause issues, see rubyjs/mini_racer#179.

Ruby's extension loading mechanism is not configurable -- there's no way to tell it to use different flags when calling dlopen. To get around this, this commit introduces introduces another extension (profiling loader) which has only a single responsibility: mimic Ruby's extension loading mechanism, but when calling dlopen use a different set of flags.

This idea was shamelessly stolen from @lloeki's work in rubyjs/mini_racer#179, big thanks!

Note that, that we know of, the profiling native extension only exposes one potentially problematic symbol: rust_eh_personality (coming from libddprof/libdatadog).

Future versions of Rust have been patched not to expose this (see rust-lang/rust#95604 (comment)) so we may want to revisit the need for this loader in the future, and perhaps delete it if we no longer require its services :)

Testing the ddtrace_profiling_loader.c's failure handling paths is a bit non-trivial, so here's how I did it manually (some logs abridged):

 # 1️⃣  Validate everything's working fine:

 $ ruby -v
 ruby 2.7.6p219 (2022-04-12 revision c9c2245c0a) [x86_64-darwin20]
 $ bundle rake clean compile
 ...
 $ DD_TRACE_DEBUG=true DD_PROFILING_ENABLED=true bundle exec ddtracerb exec ruby -e 'nil'
 D, [2022-05-06T12:17:41.554770 #19686] DEBUG -- ddtrace: [ddtrace]
 (/Users/ivo.anjo/datadog/dd-trace-rb/lib/datadog/core/configuration/components.rb:340:in `startup!') Profiling started'

 # 2️⃣  Force library load failure:

 # (Delete the library version for the Ruby I'm using)
 $ rm lib/ddtrace_profiling_native_extension.2.7.6_x86_64-darwin20.bundle
 $ DD_TRACE_DEBUG=true DD_PROFILING_ENABLED=true bundle exec ddtracerb exec ruby -e 'nil'
 W, [2022-05-06T12:19:23.891711 #21159]  WARN -- ddtrace: [ddtrace]
 (/Users/ivo.anjo/datadog/dd-trace-rb/lib/datadog/core/configuration/components.rb:345:in `startup!')
 Profiling was requested but is not supported, profiling disabled: There was an error loading the profiling native extension due to
 'Failure to load ddtrace_profiling_native_extension.2.7.6_x86_64-darwin20 due to
 dlopen(/Users/ivo.anjo/datadog/dd-trace-rb/lib/datadog/profiling/../../ddtrace_profiling_native_extension.2.7.6_x86_64-darwin20.bundle, 5):
 image not found' at '/Users/ivo.anjo/datadog/dd-trace-rb/lib/datadog/profiling/load_native_extension.rb:22:in `<top (required)>''

 # 3️⃣  Force a library incompatibility:

 # (Copy library compiled with a different Ruby version)
 $ cp lib/ddtrace_profiling_native_extension.3.2.0_x86_64-darwin20.bundle lib/ddtrace_profiling_native_extension.2.7.6_x86_64-darwin20.bundle
 $ DD_TRACE_DEBUG=true DD_PROFILING_ENABLED=true bundle exec ddtracerb exec ruby -e 'nil'
 W, [2022-05-06T12:22:24.036956 #21773]  WARN -- ddtrace: [ddtrace]
 (/Users/ivo.anjo/datadog/dd-trace-rb/lib/datadog/core/configuration/components.rb:345:in `startup!')
 Profiling was requested but is not supported, profiling disabled: There was an error loading the profiling native extension due to
 Failure to load ddtrace_profiling_native_extension.2.7.6_x86_64-darwin20 due to library was compiled and linked to a different Ruby version'
 at '/Users/ivo.anjo/datadog/dd-trace-rb/lib/datadog/profiling/load_native_extension.rb:22:in `<top (required)>'

 # 4️⃣  Force an initialization failure:

 # (Restore library clobbered in previous steps)
 $ bundle rake clean compile
 ...
 # (Rename expected initialization function)
 $ git diff -U0 lib/datadog/profiling/load_native_extension.rb
 diff --git a/lib/datadog/profiling/load_native_extension.rb b/lib/datadog/profiling/load_native_extension.rb
 index 5ea4bb32c..af964bade 100644
 --- a/lib/datadog/profiling/load_native_extension.rb
 +++ b/lib/datadog/profiling/load_native_extension.rb
 @@ -18 +18 @@ full_file_path = "#{__dir__}/../../#{extension_name}.#{RbConfig::CONFIG['DLEXT']
 -init_function_name = "Init_#{extension_name.split('.').first}"
 +init_function_name = "Init_#{extension_name.split('.').first}_kaboom
 $ DD_TRACE_DEBUG=true DD_PROFILING_ENABLED=true bundle exec ddtracerb exec ruby -e 'nil'
 W, [2022-05-06T12:27:55.706221 #23036]  WARN -- ddtrace: [ddtrace]
 (/Users/ivo.anjo/datadog/dd-trace-rb/lib/datadog/core/configuration/components.rb:345:in `startup!')
 Profiling was requested but is not supported, profiling disabled: There was an error loading the profiling native extension due to
 'Failure to load ddtrace_profiling_native_extension.2.7.6_x86_64-darwin20 due to dlsym(0x7fe144151630,
   Init_ddtrace_profiling_native_extension_kaboom): symbol not found' at
 '/Users/ivo.anjo/datadog/dd-trace-rb/lib/datadog/profiling/load_native_extension.rb:22:in `<top (required)>''

[lloeki]

LGTM!