How Does Go\'s Select Statement Handle Same-Channel Receive and Send Operations?

Same-Channel Receive and Send in Select Statements

Go's select statements offer flexible concurrency control by allowing multiple operations to be executed simultaneously. However, it raises the question of how the select statement interacts with operations that involve both receiving and sending on the same channel.

The answer lies in how Go evaluates channels and expressions within a select statement. According to the official documentation, channel operands and right-hand-side expressions of send statements are evaluated upon entering the select. This means that the expression:

case ch2 <- (<-ch1):

is effectively treated as:

case ch2 <- <something>:

where is evaluated on entering the select.

Therefore, the select statement blocks immediately on the receive operation from ch1. If ch1 has a value, the receive operation succeeds, and the send operation to ch2 is executed. If ch1 does not have a value, the select continues to wait until a value is available or another case is selected.

This behavior has the side effect of potentially consuming and discarding a value from ch1 even if the send operation to ch2 is not selected. It is important to consider this when designing concurrent programs to avoid unexpected behavior.

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