JSONRPC panic resulting in HTTP 502 "Bad Gateway"

[mdyring]

Describe the bug
JSONRPC prematurely closes the connection with this panic:

http: superfluous response.WriteHeader call from github.com/tendermint/tendermint/rpc/jsonrpc/server.(*responseWriterWrapper).WriteHeader (http_server.go:225)
Panic during RPC panic recovery: write tcp 127.0.0.1:28657->127.0.0.1:44936: i/o timeout
goroutine 8687815 [running]:
runtime/debug.Stack(0xc001b270a8, 0x126d2a0, 0xc004577bd0)
	/usr/lib/go-1.16/src/runtime/debug/stack.go:24 +0x9f
github.com/tendermint/tendermint/rpc/jsonrpc/server.RecoverAndLogHandler.func1.1(0x16a1230, 0xc0002007e0)
	/root/go/pkg/mod/github.com/tendermint/tendermint@v0.33.7/rpc/jsonrpc/server/http_server.go:162 +0x76
panic(0x126d2a0, 0xc004577bd0)
	/usr/lib/go-1.16/src/runtime/panic.go:965 +0x1b9
github.com/tendermint/tendermint/rpc/jsonrpc/server.WriteRPCResponseHTTPError(0x16a0c00, 0xc0076b2e40, 0x1f4, 0x13a26ca, 0x3, 0x1688a00, 0x16630c8, 0x0, 0x0, 0x0, ...)
	/root/go/pkg/mod/github.com/tendermint/tendermint@v0.33.7/rpc/jsonrpc/server/http_server.go:105 +0x29c
github.com/tendermint/tendermint/rpc/jsonrpc/server.RecoverAndLogHandler.func1.2(0xc0076b2e40, 0x16a60a8, 0xc0010a05e0, 0xc03902fc502625af, 0x75accab3043c, 0x1e51de0, 0xc0015de200)
	/root/go/pkg/mod/github.com/tendermint/tendermint@v0.33.7/rpc/jsonrpc/server/http_server.go:193 +0x378
panic(0x126d2a0, 0xc004577bd0)
	/usr/lib/go-1.16/src/runtime/panic.go:965 +0x1b9
github.com/tendermint/tendermint/rpc/jsonrpc/server.WriteRPCResponseHTTP(0x16a0c00, 0xc0076b2e40, 0x13a26ca, 0x3, 0x1688a00, 0xc003d4b4d8, 0xc00288a000, 0x9bdd, 0xf07a, 0x0)
	/root/go/pkg/mod/github.com/tendermint/tendermint@v0.33.7/rpc/jsonrpc/server/http_server.go:117 +0x297
github.com/tendermint/tendermint/rpc/jsonrpc/server.makeHTTPHandler.func2(0x16a0c00, 0xc0076b2e40, 0xc0015de200)
	/root/go/pkg/mod/github.com/tendermint/tendermint@v0.33.7/rpc/jsonrpc/server/http_uri_handler.go:64 +0x6f3
net/http.HandlerFunc.ServeHTTP(0xc001ee6600, 0x16a0c00, 0xc0076b2e40, 0xc0015de200)
	/usr/lib/go-1.16/src/net/http/server.go:2049 +0x44
net/http.(*ServeMux).ServeHTTP(0xc003aa2c40, 0x16a0c00, 0xc0076b2e40, 0xc0015de200)
	/usr/lib/go-1.16/src/net/http/server.go:2428 +0x1ad
github.com/tendermint/tendermint/rpc/jsonrpc/server.maxBytesHandler.ServeHTTP(0x16871e0, 0xc003aa2c40, 0xf4240, 0x16a0c00, 0xc0076b2e40, 0xc0015de200)
	/root/go/pkg/mod/github.com/tendermint/tendermint@v0.33.7/rpc/jsonrpc/server/http_server.go:240 +0xd9
github.com/tendermint/tendermint/rpc/jsonrpc/server.RecoverAndLogHandler.func1(0x16a1230, 0xc0002007e0, 0xc0015de200)
	/root/go/pkg/mod/github.com/tendermint/tendermint@v0.33.7/rpc/jsonrpc/server/http_server.go:213 +0x39a
net/http.HandlerFunc.ServeHTTP(0xc001d72f30, 0x16a1230, 0xc0002007e0, 0xc0015de200)
	/usr/lib/go-1.16/src/net/http/server.go:2049 +0x44
net/http.serverHandler.ServeHTTP(0xc000f44540, 0x16a1230, 0xc0002007e0, 0xc0015de200)
	/usr/lib/go-1.16/src/net/http/server.go:2867 +0xa3
net/http.(*conn).serve(0xc00b067360, 0x16a5858, 0xc004b2b6c0)
	/usr/lib/go-1.16/src/net/http/server.go:1932 +0x8cd
created by net/http.(*Server).Serve
	/usr/lib/go-1.16/src/net/http/server.go:2993 +0x39b

How to reproduce
Unclear. Seems to happen with a variety of queries:

2021/07/30 06:39:50 [error] 60150#60150: *176853 upstream prematurely closed connection while reading response header from upstream, client: 10.42.42.215, server: , request: "GET /emoney/api/auth/accounts/emoney14ep4hfm733sfvuaxt78mf4fkq4jzrr2mmvqe6m HTTP/1.1", upstream: "http://127.0.0.1:3317/auth/accounts/emoney14ep4hfm733sfvuaxt78mf4fkq4jzrr2mmvqe6m"
2021/07/30 06:48:55 [error] 60150#60150: *177430 upstream prematurely closed connection while reading response header from upstream, client: 10.42.43.182, server: , request: "GET /emoney/api/supply/circulating HTTP/1.1", upstream: "http://127.0.0.1:3317/supply/circulating"
2021/07/30 06:51:57 [error] 60150#60150: *177462 upstream prematurely closed connection while reading response header from upstream, client: 10.42.44.234, server: , request: "GET /emoney/api/supply/circulating HTTP/1.1", upstream: "http://127.0.0.1:3317/supply/circulating"
2021/07/30 06:51:57 [error] 60150#60150: *177776 upstream prematurely closed connection while reading response header from upstream, client: 10.42.43.182, server: , request: "GET /emoney/api/staking/pool HTTP/1.1", upstream: "http://127.0.0.1:3317/staking/pool"
2021/07/30 06:51:57 [error] 60150#60150: *177602 upstream prematurely closed connection while reading response header from upstream, client: 10.42.42.215, server: , request: "GET /emoney/api/staking/validators/emoneyvaloper1qjk97vct5gs995xmf9pk0zxt9ptpdyzpkpwt6v/delegations HTTP/1.1", upstream: "http://127.0.0.1:3317/staking/validators/emoneyvaloper1qjk97vct5gs995xmf9pk0zxt9ptpdyzpkpwt6v/delegations"
2021/07/30 06:53:10 [error] 60150#60150: *177602 upstream prematurely closed connection while reading response header from upstream, client: 10.42.42.215, server: , request: "GET /emoney/api/supply/circulating HTTP/1.1", upstream: "http://127.0.0.1:3317/supply/circulating"

However seems more prevalent when requests arrive in a bursty fashion, so could be a concurrency issue:

10.42.42.215 - - [30/Jul/2021:06:51:57 +0000] "GET /cosmoshub/health HTTP/1.1" 200 50 "-" "Amazon CloudFront"
10.42.43.182 - - [30/Jul/2021:06:51:57 +0000] "GET /emoney/health HTTP/1.1" 200 50 "-" "Amazon CloudFront"
10.42.44.234 - - [30/Jul/2021:06:51:57 +0000] "GET /emoney/api/supply/circulating HTTP/1.1" 502 166 "-" "Amazon CloudFront"
10.42.43.182 - - [30/Jul/2021:06:51:57 +0000] "GET /emoney/api/staking/pool HTTP/1.1" 502 166 "-" "Amazon CloudFront"
10.42.42.215 - - [30/Jul/2021:06:51:57 +0000] "GET /emoney/api/staking/validators/emoneyvaloper1qjk97vct5gs995xmf9pk0zxt9ptpdyzpkpwt6v/delegations HTTP/1.1" 502 166 "-" "Amazon CloudFront"
10.42.43.182 - - [30/Jul/2021:06:51:57 +0000] "GET /emoney/api/auth/accounts/emoney14ep4hfm733sfvuaxt78mf4fkq4jzrr2mmvqe6m HTTP/1.1" 200 354 "-" "Amazon CloudFront"
10.42.44.234 - - [30/Jul/2021:06:51:57 +0000] "GET /emoney/api/supply/total HTTP/1.1" 200 278 "-" "Amazon CloudFront"
10.42.42.215 - - [30/Jul/2021:06:51:57 +0000] "GET /emoney/api/distribution/community_pool HTTP/1.1" 200 339 "-" "Amazon CloudFront"
10.42.43.182 - - [30/Jul/2021:06:51:57 +0000] "GET /emoney/api/staking/validators/emoneyvaloper1zgv6tqess9q6y4cj28ldpjllrqlyzqqh80fpgu/delegations HTTP/1.1" 200 3656 "-" "Amazon CloudFront"