Summary of changes in this PR:

  * Added a new global `set_connection_limit!` function for controlling the global connection limit that will be applied to all requests
    This is one way to resolve #1033. I added a deprecation warning when passing `connect_limit` to individual requests. So usage would be:
    calling `HTTP.set_connection_limit!` and any time this is called, it changes the global value.
  * Add a try-finally in keepalive! around our global IO lock usage just for good house-keeping
  * Refactored `try_with_timeout` to use a `Threads.Condition` and `Threads.@spawn` instead of the non-threaded versions; seems cleaner
    and allows us to avoid the usage of `@async` when not needed. Note that I included a change in StreamRequest.jl however that wraps
    all the actual write/read IO operations in a `fetch(@async dostuff())` because this will currently prevent code in this task from
    migrating across threads, which is important for OpenSSL usage where error handling is done per-thread. I don't love the solution,
    but it seems ok for now.
  * I refactored a few of the stream IO functions so that we always know the number of bytes downloaded, whether in memory or written to
    an IO, so we can log them and use them in verbose logging to give bit-rate calculations
  * Ok, the big one: I rewrote the internal implementation of ConnectionPool.ConnectionPools.Pod `acquire`/`release` functions; under really
    heavy workloads, there was a ton of contention on the Pod lock. I also observed at least one "hang" where GDB backtraces seemed to indicate
    that somehow a task failed/died/hung while trying to make a new connection _while holding the Pod lock_, which then meant that no other
    requests could ever make progress. The new implementation includes a lock-free "fastpath" where an existing connection that can be re-used
    doesn't require any lock-taking. It uses a lock-free concurrent Stack implementation copied from JuliaConcurrent/ConcurrentCollections.jl (
    doesn't seem actively maintained and it's not much code, so just copied). The rest of the `acquire`/`release` code is now modeled after
    Base.Event in how releasing always acquires the lock and slow-path acquires also take the lock to ensure fairness and no deadlocks.
    I've included some benchmark results on a variety of heavy workloads [here](https://everlasting-mahogany-a5f.notion.site/Issue-heavy-load-perf-degradation-1cd275c75037481a9cd6378b8303cfb3)
    that show some great improvements, a bulk of which are attributable to reducing contention when acquiring/releasing connections during requests.
    The other key change included in this rewrite is that we ensure we _do not_ hold any locks while _making new connections_ to avoid the
    possibility of the lock ever getting "stuck", and because it's not necessary: the pod is in charge of just keeping track of numbers and
    doesn't need to worry about whether the connection was actually made yet or not (if it fails, it will be immediately released back and retried).
    Overall, the code is also _much_ simpler, which I think is a huge win, because the old code was always pretty scary to have to dig into.

src/ConnectionPool.jl

@@ -19,22 +19,21 @@ remotely closed, a connection will be reused.
 """
 module ConnectionPool
 
-export Connection, newconnection, releaseconnection, getrawstream, inactiveseconds, shouldtimeout, set_default_connection_limit!
+export Connection, newconnection, releaseconnection, getrawstream, inactiveseconds, shouldtimeout, set_default_connection_limit!, set_connection_limit!
 
 using Sockets, LoggingExtras, NetworkOptions
 using MbedTLS: SSLConfig, SSLContext, setup!, associate!, hostname!, handshake!
 using MbedTLS, OpenSSL
 using ..IOExtras, ..Conditions, ..Exceptions
 
-const default_connection_limit = Ref(8)
 const nolimit = typemax(Int)
 
-set_default_connection_limit!(n) = default_connection_limit[] = n
-
 taskid(t=current_task()) = string(hash(t) & 0xffff, base=16, pad=4)
 
 include("connectionpools.jl")
 using .ConnectionPools
+set_default_connection_limit!(n) = ConnectionPools.connection_limit[] = n
+set_connection_limit!(n) = ConnectionPools.connection_limit[] = n
 
 """
     Connection
@@ -364,15 +363,15 @@ or create a new `Connection` if required.
 function newconnection(::Type{T},
                        host::AbstractString,
                        port::AbstractString;
-                       connection_limit=default_connection_limit[],
+                       connection_limit=nothing,
                        forcenew::Bool=false,
                        idle_timeout=typemax(Int),
                        require_ssl_verification::Bool=NetworkOptions.verify_host(host, "SSL"),
                        kw...) where {T <: IO}
     return acquire(
             getpool(T),
             (T, host, port, require_ssl_verification, true);
-            max_concurrent_connections=Int(connection_limit),
+            max_concurrent_connections=connection_limit,
             forcenew=forcenew,
             idle_timeout=Int(idle_timeout)) do
         Connection(host, port,
@@ -383,16 +382,21 @@ function newconnection(::Type{T},
     end
 end
 
-releaseconnection(c::Connection{T}, reuse) where {T} =
+function releaseconnection(c::Connection{T}, reuse) where {T}
+    c.timestamp = time()
     release(getpool(T), connectionkey(c), c; return_for_reuse=reuse)
+end
 
 function keepalive!(tcp)
     Base.iolock_begin()
-    Base.check_open(tcp)
-    err = ccall(:uv_tcp_keepalive, Cint, (Ptr{Nothing}, Cint, Cuint),
-                                          tcp.handle, 1, 1)
-    Base.uv_error("failed to set keepalive on tcp socket", err)
-    Base.iolock_end()
+    try
+        Base.check_open(tcp)
+        err = ccall(:uv_tcp_keepalive, Cint, (Ptr{Nothing}, Cint, Cuint),
+                                            tcp.handle, 1, 1)
+        Base.uv_error("failed to set keepalive on tcp socket", err)
+    finally
+        Base.iolock_end()
+    end
     return
 end
 
@@ -431,7 +435,7 @@ function getconnection(::Type{TCPSocket},
             return if connect_timeout > 0
                 tcp = Sockets.TCPSocket()
                 Sockets.connect!(tcp, addr, p)
-                try_with_timeout(() -> checkconnected(tcp), connect_timeout, () -> close(tcp)) do
+                try_with_timeout(connect_timeout) do
                     Sockets.wait_connected(tcp)
                     keepalive && keepalive!(tcp)
                 end
@@ -442,6 +446,7 @@ function getconnection(::Type{TCPSocket},
                 tcp
             end
         catch e
+            close(tcp)
             lasterr = e isa TimeoutError ? ConnectTimeout(host, port) : e
             continue
         end
@@ -545,8 +550,13 @@ function sslupgrade(::Type{IOType}, c::Connection{T},
     # if the upgrade fails, an error will be thrown and the original c will be closed
     # in ConnectionRequest
     tls = if readtimeout > 0
-        try_with_timeout(() -> shouldtimeout(c, readtimeout), readtimeout, () -> close(c)) do
-            sslconnection(IOType, c.io, host; require_ssl_verification=require_ssl_verification, kw...)
+        try
+            try_with_timeout(readtimeout) do
+                sslconnection(IOType, c.io, host; require_ssl_verification=require_ssl_verification, kw...)
+            end
+        catch
+            close(c)
+            rethrow()
         end
     else
         sslconnection(IOType, c.io, host; require_ssl_verification=require_ssl_verification, kw...)

src/Exceptions.jl

@@ -25,41 +25,22 @@ macro $(:try)(exes...)
 end
 end # @eval
 
-function try_with_timeout(f, shouldtimeout, delay, iftimeout=() -> nothing)
-    @assert delay > 0
-    cond = Condition()
-    # execute f async
-    t = @async try
-        notify(cond, f())
-    catch e
-        @debugv 1 "error executing f in try_with_timeout"
-        isopen(timer) && notify(cond, e, error = true)
-    end
-    # start a timer
-    timer = Timer(delay; interval=delay / 10) do tm
+function try_with_timeout(f, timeout)
+    ch = Channel(0)
+    timer = Timer(tm -> close(ch, TimeoutError(timeout)), timeout)
+    Threads.@spawn begin
         try
-            if shouldtimeout()
-                @debugv 1 "❗️  Timeout: $delay"
-                close(tm)
-                iftimeout()
-                notify(cond, TimeoutError(delay), error = true)
-            end
+            put!(ch, $f())
         catch e
-            @debugv 1 "callback error in try_with_timeout"
-            close(tm)
-            notify(cond, e, error = true)
+            close(ch, CapturedException(e, catch_backtrace()))
+        finally
+            if isopen(timer)
+                close(timer)
+                notify(event)
+            end
         end
     end
-    try
-        res = wait(cond)
-        @debugv 1 "try_with_timeout finished with: $res"
-        res
-    catch e
-        @debugv 1 "try_with_timeout failed with: $e"
-        rethrow()
-    finally
-        close(timer)
-    end
+    return take!(ch)
 end
 
 abstract type HTTPError <: Exception end

src/Streams.jl

@@ -281,7 +281,7 @@ end
 
 @noinline function bufcheck(buf::Base.GenericIOBuffer, n)
     requested_buffer_capacity = (buf.append ? buf.size : (buf.ptr - 1)) + n
-    requested_buffer_capacity > length(buf.data) && throw(ArgumentError("Unable to grow response stream IOBuffer large enough for response body size"))
+    requested_buffer_capacity > length(buf.data) && throw(ArgumentError("Unable to grow response stream IOBuffer $(length(buf.data)) large enough for response body size: $requested_buffer_capacity"))
 end
 
 function Base.readbytes!(http::Stream, buf::Base.GenericIOBuffer, n=bytesavailable(http))
@@ -299,19 +299,25 @@ function Base.readbytes!(http::Stream, buf::Base.GenericIOBuffer, n=bytesavailab
     return n
 end
 
-Base.read(http::Stream, buf::Base.GenericIOBuffer=PipeBuffer()) = take!(readall!(http, buf))
+function Base.read(http::Stream, buf::Base.GenericIOBuffer=PipeBuffer())
+    readall!(http, buf)
+    return take!(buf)
+end
 
 function readall!(http::Stream, buf::Base.GenericIOBuffer=PipeBuffer())
+    n = 0
     if ntoread(http) == unknown_length
         while !eof(http)
-            readbytes!(http, buf)
+            n += readbytes!(http, buf)
         end
     else
+        # even if we know the length, we still need to read until eof
+        # because Transfer-Encoding: chunked comes in piece-by-piece
         while !eof(http)
             readbytes!(http, buf, ntoread(http))
         end
     end
-    return buf
+    return n
 end
 
 function Base.readuntil(http::Stream, f::Function)::ByteView

src/clientlayers/ConnectionRequest.jl

@@ -91,7 +91,7 @@ function connectionlayer(handler)
                     target_url = URI(target_url, port=80) # if there is no port info, connect_tunnel will fail
                 end
                 r = if readtimeout > 0
-                    try_with_timeout(() -> shouldtimeout(io, readtimeout), readtimeout, () -> close(io)) do
+                    try_with_timeout(readtimeout) do
                         connect_tunnel(io, target_url, req)
                     end
                 else
@@ -110,6 +110,9 @@ function connectionlayer(handler)
             stream = Stream(req.response, io)
             return handler(stream; readtimeout=readtimeout, kw...)
         catch e
+            if e isa TaskFailedException
+                e = e.task.result
+            end
             @debugv 1 "❗️  ConnectionLayer $e. Closing: $io"
             shouldreuse = false
             @try Base.IOError close(io)

src/clientlayers/MessageRequest.jl

@@ -4,6 +4,7 @@ using URIs
 using ..IOExtras, ..Messages, ..Parsers, ..Exceptions
 using ..Messages, ..Parsers
 using ..Strings: HTTPVersion
+using LoggingExtras
 
 export messagelayer
 
@@ -23,6 +24,7 @@ function messagelayer(handler)
     return function(method::String, url::URI, headers, body; copyheaders::Bool=true, response_stream=nothing, http_version=HTTPVersion(1, 1), kw...)
         req = Request(method, resource(url), mkreqheaders(headers, copyheaders), body; url=url, version=http_version, responsebody=response_stream)
         local resp
+        start_time = time()
         try
             resp = handler(req; response_stream=response_stream, kw...)
         catch e
@@ -38,6 +40,12 @@ function messagelayer(handler)
                     write(resp.body, resp.request.context[:response_body])
                 end
             end
+            if @isdefined(resp)
+                end_time = time()
+                bytes = get(resp.request.context, :nbytes, 0)
+                mbits_per_second = bytes == 0 ? 0 : (((8 * bytes) / 1e9) / (end_time - start_time))
+                @debugv 1 "Request complete with bandwidth: $(mbits_per_second) Gbps"
+            end
         end
     end
 end

src/clientlayers/RetryRequest.jl

@@ -77,6 +77,7 @@ function retrylayer(handler)
 end
 
 isrecoverable(ex) = true
+isrecoverable(ex::ConnectError) = ex.error isa Sockets.DNSError && ex.error.code == EAI_AGAIN ? false : true
 isrecoverable(ex::StatusError) = retryable(ex.status)
 
 function _retry_check(s, ex, req, check)

src/clientlayers/StreamRequest.jl

@@ -17,7 +17,11 @@ immediately so that the transmission can be aborted if the `Response` status
 indicates that the server does not wish to receive the message body.
 [RFC7230 6.5](https://tools.ietf.org/html/rfc7230#section-6.5).
 """
-function streamlayer(stream::Stream; iofunction=nothing, decompress::Union{Nothing, Bool}=nothing, kw...)::Response
+function streamlayer(stream::Stream; kw...)::Response
+    return fetch(@async _streamlayer(stream; kw...))
+end
+
+function _streamlayer(stream::Stream; iofunction=nothing, decompress::Union{Nothing, Bool}=nothing, kw...)::Response
     response = stream.message
     req = response.request
     io = stream.stream
@@ -132,6 +136,7 @@ end
 const IOBuffers = Union{IOBuffer, Base.GenericIOBuffer{SubArray{UInt8, 1, Vector{UInt8}, Tuple{UnitRange{Int64}}, true}}}
 
 function readbody!(stream::Stream, res::Response, buf_or_stream)
+    n = 0
     if !iserror(res)
         if isbytes(res.body)
             if length(res.body) > 0
@@ -144,29 +149,31 @@ function readbody!(stream::Stream, res::Response, buf_or_stream)
                     # if it's a BufferStream, the response body was gzip encoded
                     # so using the default write is fastest because it utilizes
                     # readavailable under the hood, for which BufferStream is optimized
-                    write(body, buf_or_stream)
+                    n = write(body, buf_or_stream)
                 elseif buf_or_stream isa Stream
                     # for HTTP.Stream, there's already an optimized read method
                     # that just needs an IOBuffer to write into
-                    readall!(buf_or_stream, body)
+                    n = readall!(buf_or_stream, body)
                 else
                     error("unreachable")
                 end
             else
                 res.body = read(buf_or_stream)
+                n = length(res.body)
             end
         elseif res.body isa Base.GenericIOBuffer && buf_or_stream isa Stream
             # optimization for IOBuffer response_stream to avoid temporary allocations
-            readall!(buf_or_stream, res.body)
+            n = readall!(buf_or_stream, res.body)
         else
-            write(res.body, buf_or_stream)
+            n = write(res.body, buf_or_stream)
         end
     else
         # read the response body into the request context so that it can be
         # read by the user if they want to or set later if
         # we end up not retrying/redirecting/etc.
         res.request.context[:response_body] = read(buf_or_stream)
     end
+    res.request.context[:nbytes] = n
 end
 
 end # module StreamRequest

src/clientlayers/TimeoutRequest.jl

@@ -16,8 +16,7 @@ function timeoutlayer(handler)
             # skip
             return handler(stream; kw...)
         end
-        io = stream.stream
-        return try_with_timeout(() -> shouldtimeout(io, readtimeout), readtimeout, () -> close(io)) do
+        return try_with_timeout(readtimeout) do
             handler(stream; kw...)
         end
     end

src/concurrentstack.jl

@@ -0,0 +1,35 @@
+mutable struct Node{T}
+    value::T
+    @atomic next::Union{Node{T},Nothing}
+end
+
+Node{T}(value::T) where {T} = Node{T}(value, nothing)
+
+mutable struct ConcurrentStack{T}
+    @atomic next::Union{Node{T},Nothing}
+end
+
+ConcurrentStack{T}() where {T} = ConcurrentStack{T}(nothing)
+
+function Base.push!(stack::ConcurrentStack{T}, v) where {T}
+    v === nothing && throw(ArgumentError("cannot push nothing onto a ConcurrentStack"))
+    v = convert(T, v)
+    node = Node{T}(v)
+    next = @atomic stack.next
+    while true
+        @atomic node.next = next
+        next, ok = @atomicreplace(stack.next, next => node)
+        ok && break
+    end
+    return stack
+end
+
+function Base.pop!(stack::ConcurrentStack)
+    while true
+        node = @atomic stack.next
+        node === nothing && return nothing
+        next = @atomic node.next
+        next, ok = @atomicreplace(stack.next, node => next)
+        ok && return node.value
+    end
+end