In the test examples and real usage scenarios, 'mbedtls_net_free' is called after 'mbedtls_net_connect' fails, which will cause the problem of double close the same fd. It is possible to close this closed fd which has been applied by other link.

Description

hi all. We found that in the scenario of sudden network disconnection, ‘mbedtls_net_connect’ will close fd after the connection fails, and then call ‘mbedtls_net_free’ to close it again. There is a risk of double close here. At first, we suspected that it was used incorrectly, but after referring to the implementation and case in the programs directory, we did not find that it was a usage error. Therefore, we suggest to mark fd as unavailable after closing it to prevent fd from being used by other programs during the interval between 'mbedtls_net_connect' and 'mbedtls_net_free'.

PR checklist

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• changelog provided
• development PR provided net/mbedtls_net_connect: Preventing double close problem #9711
• framework PR not required
• 3.6 PR here
• 2.28 PR provided [Backport 2.28] net/mbedtls_net_connect: Preventing double close problem #9715
• tests not required because: minor change, testing would require help from the OS

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