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Fix possible 100% CPU loop in CivetWeb #2882

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Fix possible 100% CPU loop in CivetWeb #2882

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11 changes: 11 additions & 0 deletions src/webserver/civetweb/civetweb.c
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Original file line number Diff line number Diff line change
Expand Up @@ -479,6 +479,13 @@ _civet_safe_clock_gettime(int clk_id, struct timespec *t)
#define SOCKET_TIMEOUT_QUANTUM (2000) /* in ms */
#endif

/* Non-blocking mbedTLS operations can repeatedly return WANT_READ/WRITE
* without making application-visible progress. Add a tiny backoff to avoid
* tight retry loops on idle HTTPS keep-alive connections. */
#if !defined(MG_MBEDTLS_WANT_RETRY_DELAY_MS)
#define MG_MBEDTLS_WANT_RETRY_DELAY_MS (5)
#endif

/* Do not try to compress files smaller than this limit. */
#if !defined(MG_FILE_COMPRESSION_SIZE_LIMIT)
#define MG_FILE_COMPRESSION_SIZE_LIMIT (1024) /* in bytes */
Expand Down Expand Up @@ -6488,6 +6495,10 @@ pull_inner(FILE *fp,
if ((nread == MBEDTLS_ERR_SSL_WANT_READ)
|| (nread == MBEDTLS_ERR_SSL_WANT_WRITE)
|| nread == MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS) {
/* The socket may stay poll-readable while mbedTLS still has
* no application data to return. Without a small backoff,
* higher-level request loops can devolve into a busy spin. */
mg_sleep(MG_MBEDTLS_WANT_RETRY_DELAY_MS);
nread = 0;
} else {
fprintf(stderr, "SSL read failed, error %d\n", nread);
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5 changes: 5 additions & 0 deletions src/webserver/civetweb/mod_mbedtls.inl
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Expand Up @@ -321,6 +321,11 @@ mbed_ssl_handshake(mbedtls_ssl_context *ssl)
&& rc != MBEDTLS_ERR_SSL_ASYNC_IN_PROGRESS) {
break;
}

/* Accepted HTTPS sockets are non-blocking. If the handshake cannot
* progress yet, yield briefly so we do not spin a worker thread at
* 100% CPU waiting for the peer. */
mg_sleep(MG_MBEDTLS_WANT_RETRY_DELAY_MS);
}

#if MBEDTLS_VERSION_NUMBER >= 0x03000000
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