net/http: disable 100 continue status after handler finished

[AlexanderYastrebov]

When client supplies "Expect: 100-continue" header,
server wraps request body into expectContinueReader
that writes 100 Continue status on the first body read.

When handler acts as a reverse proxy and passes incoming
request (or body) to the client (or transport) it may happen
that request body is read after handler exists which may
cause nil pointer panic on connection write if server
already closed the connection.

This change disables write of 100 Continue status by expectContinueReader
after handler finished and before connection is closed.

It also fixes racy access to w.wroteContinue.

Fixes #53808
Updates #46866

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Message from Alexander Yastrebov:

Patch Set 1:

(2 comments)

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Message from Alexander Yastrebov:

Patch Set 1:

(1 comment)

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Message from Alexander Yastrebov:

Patch Set 2:

(2 comments)

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Message from Damien Neil:

Patch Set 3: Commit-Queue+1

(5 comments)

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Patch Set 3:

Dry run: CV is trying the patch.

Bot data: {"action":"start","triggered_at":"2024-04-01T17:48:53Z","revision":"5fc80acd5889257d70d63a661a1dc954e9418864"}

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Message from Damien Neil:

Patch Set 3: -Commit-Queue

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Patch Set 3:

This CL has passed the run

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Patch Set 3: LUCI-TryBot-Result+1

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Message from Alexander Yastrebov:

Patch Set 4:

(5 comments)

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Message from Alexander Yastrebov:

Patch Set 4:

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Message from Alexander Yastrebov:

Patch Set 5:

(1 comment)

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Message from Alexander Yastrebov:

Patch Set 6:

(1 comment)

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