How can I check if my function is called with input from a pipe?

Question

I usually use if [ -t 0 ] to test stdin and if [ -t 1 ] to see if stdin and stdout are TTY's, and if they aren't I assume that they're pipes. I recently learned that that is a bad assumption:

function context()
{
    if [ -t 0 ]
    then
        if [ -t 1 ]
        then
            echo "no pipes"
        else
            echo "pipe out only"
        fi
    else
        if [ -t 1 ]
        then
            echo "pipe in only"
        else
            echo "pipe in and out"
        fi
    fi
}

echo "No Loop:"        # these cases work as desired
context
echo 'f' | context
echo 'f' | context | cat
context | cat

echo
echo "Loop:"
echo $'foo
bar' | while read x
do
    context             # I want this to say "no pipes"
    echo 'f' | context
done

The output of the above bash is this:

No Loop:
no pipes
pipe in only
pipe in and out
pipe out only

Loop:
pipe in only
pipe in only
pipe in only
pipe in only

How can I change the definition of context to so that the 5th and 7th calls print "no pipes"?

That is, rather than testing whether stdin is not a terminal (which causes the problem) I'd like to test whether stdin for this particular function happens to be a pipe. I see that -p checks to see if it's a named pipe, but I want this for anonymous pipes.

Edit

The actual use case involves a function we'll call theDb in the place of context. Ideally the two cases below would be equivalent regardless of context:

cat file_with_some_sql | theDb
theDb "sql goes here"

Answer 1

The reason "pipe in only" appears four times is in fact because the context function is working correctly, and the echo $'foo bar' | while read x etc. is a pipe -- everything in the while loop is getting input from a pipe. To get the desired output, change the while loop to a for loop, and don't pipe anything to that for loop:

for x in foo bar; do  context ;  echo 'f' | context; done

Output:

no pipes
pipe in only
no pipes
pipe in only

To show where the OP code went wrong, just pipe something into the for loop, and see what happens:

echo | for x in foo bar; do  context ;  echo 'f' | context; done

Output:

pipe in only
pipe in only
pipe in only
pipe in only

A novice might (incorrectly) suppose that while read x consumes all of standard input, but that is not necessarily so. Consider this example, with a read inside a while loop:

printf '%s\n' foo bar baz buzz | 
while read x; do echo $x; read y; echo ${y^^} ;done

Output:

foo
BAR
baz
BUZZ